Happy to!
First of all, if you choose a Radian do NOT use LATCH to install it. It WILL loosen. It may take days or weeks, but it will happen (it's happened to both of my Radians). The seatbelt install is more reclined, so I miss the LATCH install, but it simply isn't safe if it doesn't stay tight.
Second, where are you going to put the seat? and, how long do you want to rf your child?
With you and your DH being so tall, between those two, I'd rec the taller shell of the Radian to keep your LOs rfing longer, but, if you have to install behind the driver, or even the passenger, front seat room may be tight. My DH and I are fine w/ it behind the passenger, but neither of us is overly tall. The nice thing abt VWs is that the room up front is quite deceptive. There is more room than it seems. Neither DH nor I can drive w/ it behind the driver. However, it fits perfectly between the front seats, so if you're doing a middle install, you're golden.
We CAN drive w/ a Blvd (same shell as the classic marathon, but the new Britaxes take up even less space) behind the driver and I drive fairly far back. There is no prohibition in our manual against bracing, but you should check yours.
As for the rfing tether, b/c of the way the track is attached to the frame, it's hard to loop the anchor point there. If you do, it severely limits how far back you can move the front seat. My DH and I can manage in the passenger seat, but we chose to use the TFP (anti-rebound bar instead of rfing tether) behind the driver to give us more room to move the front seat back. I doubt you could tether there because you both are taller than both of us. You can simply not use the rfing tether (it is optional on both seats). This is an issue w/ both seats when using a rfing tether. Neither seat has a prob w/ a FFing tether.
There is another option for rfing tethers; that is looping around the front seat buckle stalk. I don't do that b/c the seatbelts have pre-tensioners, and I read on here that tethering to that may (will?) impact how the pre-tensioners work. I'm still looking for another place to attach the anchor point. I haven't found one yet. If I've misunderstood, I hope a tech will clarify!
BTW, if your kid(s) are lightweights, I'd rec looking at a TFP. They fit in the Jetta Sportwagen really well. If you kid(s) are more average weight and tall, you might also want to look at a Complete Air. It doesn't rf tether, but it might fit your needs (esp if you can't find a way to rf tether anyway).
HTH!