The Graco CarGo has a regular 40 pound harnessed weight limit. The only seats with a higher harnessed weight limit are the Britax Marathon, Decathlon, Boulevard, Regent, the Fisher Price Safe Voyage Deluxe, the Sunshine Kids Radian and the Cosco/Safety 1st Apex. So higher weight limit choices are pretty limited, and the only budget option (less than $180) is the Apex, at $130. However, it is only acceptable to use this particular seat in vehicles with tall seatbacks or headrests. There needs to be vehicle seatback or headrest at least up to the tips of the child's ears at all times. In other words, the carseat does not provide adequate head protection.
We don't recommend the ComfortSport (or any Evenflo convertibles), first of all, because of it's low (30 pound) rear-facing weight limit. Rear-facing longer is safer, the American Academy of Pediatrics has recommended for years to keep kids rear-facing until they max out the rear-facing limits of the convertible seat. Research shows that kids up to age 2 are four times more likely to die in a crash if they are forward-facing. Kids in Sweden rear-face to 55 pounds and in a five year timespan only 9 children rear-facing were killed in car crashes, and those involved severe intrusion into the vehicle. In stark contrast, car crashes killed 1,638 children ages 14 years and younger in the year 2004 ALONE, and approximately 214,000 were injured. That’s an average of 5 deaths and 586 injuries each day.
See this page for more rear-facing information:
http://www.cpsafety.com/articles/stayrearfacing.aspx
Be sure to watch the crash test video comparisons of RF and FF.
In addition to the low rear-facing weight limit, the ComfortSport has the lowest top harness slot height of all convertible carseats on the market. This is bad because a seat is outgrown forward-facing when the tips of the child's ears go over the top of the seat shell OR when the child's shoulders go over the top harness slots. Stated height limit in the manual doesn't matter; all kids are built differently, some are all torso, others are all legs, others are evenly proportioned. Long-torsoed and/or lightweight kids will outgrow seats with low top harness slots long before reaching 40 pounds.
Example: my dd outgrew her Cosco Alpha Omega by height at 38 inches tall and 33 pounds. She would have already been too tall for the ComfortSport by 36 inches tall and 30 pounds.
A ComfortSport costs roughly $80-$100, for maybe 3 years' worth of use (and that's stretching it for almost every child), then another seat with taller top slots is needed to actually get a child to at least 40 pounds in a harness. So, add the cost of another harnessed seat ($80-$120 for a CarGo), and you've spent $160-$220 to keep a child harnessed to 40 pounds.
Going with a Britax 65 pound weight limit convertible for $250 means you don't have to buy another seat AND it means your child will get to stay in a harness not just to 40 pounds, but well beyond 40 pounds. It puts off the booster stage by another 2-3 years for most kids (and if you'd like crash test video comparisons of harnessed seats vs. boosters, I have those, too).
The Britax price tag is partly because you can use the seat for so much longer than any other seat, and partly because of the extra level of comfort they provide, and also their ease-of-use. Compared to any other carseat I've used (Graco ComfortSport, Cosco Alpha Omega, Cosco Touriva, Century SmartFit, Nania Airway, Fisher Price Futura, Evenflo Chase, Graco CarGo and Britax Husky, I'm sure I've forgotten some), the Britax is BY FAR the easiest for me to use and my daughter's favorite, by far.
Still, there are other options that will work, and keep your children safe. Britax seats may have added features, but they all pass the same minimum federal standards. We have no way of knowing if any one exceeds the other. We just have to do our research on things like harnessed weight limits, rear-facing limits, top harness slot heights and ease-of-use.
So you have your 3yo in a CarGo in the main vehicle, she is 39.5 inches and 29 pounds? And in a ComfortSport in the IL's vehicle? So you want a new seat for the IL's car for the 3yo?
And baby is in the Portabout in your car, and probably has a base in the IL's car for now?
3yo may already be too tall (shoulders above top slots) for the ComfortSport. I'd have no problems with the IL's using the ComfortSport for the baby in their car, provided they can use it properly rear-facing. Sometimes the lever adjuster is wedged into the vehicle seatback and next to impossible to access. It's important that they can install it and use it properly for it to be safe for her.
A CarGo would be a fine second seat for the 3yo, BUT you don't like it as a booster, so consider the Evenflo Bolero or Generations. I know you don't like Evenflo, and it will feel a little flimsy compared to the CarGo, probably, but it is a safe seat, with tall top slots (about the same as the CarGo), and it makes a good long-term booster when the time comes.
If you love the CarGo and would rather get another one of those, that's fine, too (Evenflo, Graco and Recaro are the only 40 pound harnessed weight limit combination harnessed/booster seats we recommend because the others have short top harness slots), but I thought I'd mention something that makes a good booster. The Recaro Young Sport is the absolute best combo seat, but it is pricey ($250).
Okay, I've gone on for way too long, LOL. Hopefully I explained myself well enough, if not, I'll post back later.