So here's most of DH's response. I tried to edit for clarity and spelling. He tends to not mince words so my apologies if anything comes off harshly. Warning...this is long.
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I have no reason to sugar coat things. This is coming from a guy, I was 16 at one point, I had friends who were 16. I now spend a lot of my life researching everything about vehicles and have a number of friends and acquaintances who have, and do own every range of vehicle up to and including owning multiple vehicles in the six digit range.
Okay, first thing to consider, your kids are 16ish. The people they hang around with, friends or not are the same age. Your sons may be very mature, but just keep in mind who their friends are. Your kids may know how to drive, but the road is filled with other people that think they know how to drive as well. Things happen, minor or severe. There are going to be people with shopping carts in parking lots, people will park beside them and swing their doors open too far, people walk by and rub against the car, scratching it. There are also people and many kids that have jealousy issues.
That being said first thing to think of is the capability of each car. Most people now that own typical Toyota Corollas, Honda Civics, Chevy Cobalts or Dodge Calibers may love them and think they are quick and fun, but being realistic they have under 200 horsepower and most have 2/3'rds that in torque. They are not all that fast so for the most part people and kids can put their foot to the floor and it is still fairly easy to keep the vehicle under control. You take people that drive something like that and put them in a 350Z, Audi TT and they do the same thing, and it gets them into a lot of trouble. I seriously, seriously suggest that if you do want them to have something like a 350 or a TT that you put the money and make them put the time into taking a track racing/school course. It teaches students how to properly take corners, estimate speed, properly brake and most of all learn the capabilities of the vehicles. For the most part these schools are a good idea anyways, but lets be realistic, if you put your kids in a Yaris, they are not going to be getting into too much trouble.
Be careful getting new, young drivers rear wheel drive cars. Without experience in powering corners they have very little tolerance for mistakes and could get your kids into some trouble if they decide to show off. Also, something to think about is how other drivers are around vehicles such as TT's, 350's, and RX8's. They will have an endless supply of people trying to get them to street race, believe me on this. Because of stuff like this the police also know, target, and profile these cars. For a young driver, the less attention that gets drawn to them the better.
This is a HUGE list and I know people who own them, or have driven or been in most of this list. I will try to keep this short but go into very basic positives and negatives of each. If you have anymore direct questions please ask and I can go into detail when you have narrowed down this list. I will also try to list these positives and negatives based on an outsiders view of a 16 year old having these vehicles.
VW GTI - VW's are "cool", I can't dispute that. Thing to be careful of is that VW (Who also owns Audi) is notoriously plagued with electrical issues. Something to also be careful of is that a lot of VW’s require a very specific type engine oil (not a brand, not weight, but a type of oil). Not just anything regular you pull off the shelf at Canadian tire, Walmart ect. As long as you are aware of this it is fine. VW’s reputation with reliability is what has kept me away from owning any of the vehicles they have produced in the past decade.
The other VW's that are on the list still fall under this. The information from reputable long term reliability studies is out there (such as JDPower's lists) VW doesn't even rate in the top 22 for overall manufacturer reliability. It rates below GMC and Chevrolet, and well below industry average. To put it as simply as I can, you will either get a car that is perfect and won't have anything wrong, or you will get a gremlin plagued car that you will never be able to fully fix. I don't mean to diss VW, but the information is out there. However if you are adamant about a VW I strongly urge you to consider the VW Jetta TDI. As long as it is taken care of (and the proper oil used) the motor itself will last a long time, and the fuel economy is through the roof. As well overall the TDi Jetta is a fun car. And of the list of the VW's that you named, that would be my only choice.
Audi TT - Older body style is not well liked among car enthusiasts. The new body style of the previous year are excellent. Old and new body styles considered I have nothing really bad to say about TT's. Handle well, fun cars. However the TT does not come equipped with the standard "quatro" technology AWD system. It is actually the Haldex system AWD, which HEAVILY favors the front wheels. Nothing wrong with it, but just consider you are not getting a true AWD car with a 50/50, or even a 60/40 Torque split. Haldex system delivers 97.5% of the power to the front, and only 2.5 to the rear. So in some cases, if you are considering a TT a sporty car it is pretty much considered a FWD car and is a little safer for a first time driver.
Audi is outside of the top ten for reliability but isn't in a scary spot. However, like all German autos it is rather costly to repair and maintain. Service intervals can be expensive. If you are leaving all repairs and maintenance up to your kids, I hope they have good summer jobs. For vehicles like Audi you NEED to follow the service intervals, especially transmission and differential fluid changes. All basic stuff that people should be doing anyways (following manual for service intervals) So as long as that is kept up, regular fluid changes, you are all good. You may or may not consider it a downside, but TT's are basically only two seaters, so your sons can have one person riding with them and that is it. Little to no trunk space either.
Nissan 350Z - No downside to this car really as far as a drivers car goes. It handles very well, it is reliable, looks good. It is rear wheel drive so like I stated above for first time drivers I would strongly urge you to have them take a track driven school with it to learn how to handle it. I also mentioned above that for young drivers this car could be trouble. Even if your kids are angels, other people are going to be trying to get them to race constantly. All things considered the 350Z is a street track/race car.
Mazda RX8. - Overall another good street track/race car. As long as you can get past the looks they perform well overall. You mentioned you know it is a 1.8L. But don't be mistaken these are very quick cars and they make decent power, I will explain why below. Same as the 350Z, they have a reputation about the drivers as street racers.
Something to note is that this isn't a standard piston combustion engine, these are rotary engine cars. These do not fall under the same rules as standard flat/inline 4cyl cars. The rotary engine has a very good, strong power band, but the design itself is self destructive. There are a lot of wear on these engines and thus the recommendation is an oil change every 3000Km. This is regardless of oil used, as I said they don't fall under the same rules as a piston driven engine. Also you will hardly ever see a rotary engine "running like new" over 150,000Km. Life is very hard for those motors, but while they last they are a lot of fun and sound fantastic.
The next list comes off to me as a much more practical first car for 16 year olds.
Versa, Yaris, Mazda 2. I will start off with these. All good choices. All available in 4 doors, all 4 cyl, all available with manual. Overall very practical, fun cars and a little bit harder to get yourself into trouble with. Of all the choices the Mazda 2 is the newest platform (to North America) It is based off the Ford Fiesta which for many, many years has been a European model. Very cool and two thumbs up for me. Features and looks alone it is cool and would be a safe bet. I am sure your kids would be happy with it. The Versa in my opinion isn't the best looker, but reliability and everything else it has little to fault it for in the price range.
Corolla, Sentra, Matrix. I will start off with the Sentra. Reliable, fun 2L car but to me the looks of the newer models are not as appealing, the only thing I can fault the car for really. A slightly bigger sedan then the three choices above, more power, more practicality. These three are all kind of the same idea, they are in the next price range up from the three above, more room, more power, at the price of fuel economy. But being realistic the Versa, Mazda 2, and the Yaris are going to be getting in the mid 30 US MPG's on the highway, the Corolla, Sentra, Matrix (FWD variant, I will get into the AWD after) are going to be in the low 30's. Not that much of a difference depending on how fuel conscious you are.
The Matrix is the only one of these to be offered in an AWD option. The AWD system used is the same as the Rav4, and is basically front wheel drive until the front wheels slip and it can go to a maximum of 55/45 torque spit. It isn't a full time AWD system, but it is a decent one. You will lose fuel economy, as I believe the AWD Matrix gets about 28Mi/G. Of all the choices the Matrix is the most infinitely useful. The hatchback with fold down seats means room for pretty much anything. If your sons are into camping, biking, whatever. The inside of the vehicle can probably be configured to fit it.
Any of the six from this section are great choices, cheap to run, fix, reliable even if some service intervals are skipped here and there.
Next, your SUV's/crossovers.
Subaru Forester - I may be biased, I own a 2005 Subaru WRX wagon. Love it to death. I am assuming you're looking at the non turbo XT model? Either way, great all around vehicle, useful, practical. The non turbo versions lack a little bit of the power for the size of the vehicle but it isn't that bad. Of any of the vehicles you listed the Subaru comes with a true, full time AWD system, it is the best of any of the AWD systems from anything in your list. Any sort of winter conditions don't mean anything to this car. Previous generations have suffered from being somewhat bland, but in the recent model years from 2008 and up the interior is quite a bit nicer.
BMW X3 - A quote from someone I know who owns a BMW M3. "Nothing will get you to the mechanic faster or in better style than a BMW" As far as a drivers car goes, the X3 does nothing well, it isn't fast, it doesn't handle well, and it doesn't get good fuel economy. However it is comfortable and looks good. Any sort of BMW service is going to hit you hard in the pocket book. However as far as being 16 goes, having an X3 would be pretty awesome. Girls would like it too.
Nissan Rogue - Another vehicle I can't fault for being what it is really. Available AWD makes it a good choice. I think they look good, reliable and pretty good on gas for being a small SUV
Mazda CX-7 - competition for the Rogue, Although I believe the CX 7 is a little bigger. It shares a platform with the Ford Edge. Same as the Rogue you can get it with AWD as well. Really between the Rogue and the CX7 it comes down to which one they like the looks of, which one they like the interior on the most, and which one you can get the best price on.
Toyota Venza - You are considering getting this? Nothing wrong with it overall, it is considered a higher end of the Toyota line, more luxury SUV. All things considered it comes off as an adult's vehicle. In my opinion it is overpriced for what you get, and in that price range you open up a lot of other options that are cooler, especially to teenage sons.
Ford Escape/Tribute - Can't fault them. My first vehicle was a 4 door Suzuki Sidekick and for comparisons sake they are near the same thing. That was probably some of the most fun I have had during my teenage years was driving it. Fun for some off-roading adventures, enough room for stuff and friends and they are not powerful enough to break anything. Reliable cars, decently priced and easy to fix. Really good choices. If they already drove them and liked them I can think of no reasons to tell you to not get it.
Porsche Cayenne
You may be looking at between $200 and $300 for an oil change. For any large service intervals don't be surprised when the bill reaches past the four digit mark. Everything is more for a Porsche. If you can afford to buy it and maintain it go for it, they are a complete pleasure to drive. But the trick with any high end vehicle is keeping the maintenance up to date. I will put it this way, the price of the vehicle being ignored if you compared it to say, a Yaris. Just to give you an example from a well known parts distributor where I buy my brake pads from - the cost of very good set of front break pads for a Yaris was $90, the same brand for the Cayenne were $190.
The VW/Audi's - I already talked about the VW's above, The Audi's are the same as the X3 really. Nice looking, comfortable. But not good on gas, expensive to maintain and fix. Not horrible choices, but when looking across the list there are much safer choices for the money.