Since your child is a peanut you definitely have a lot more freedom in choosing a carseat (I can't get britaxes because my kids outgrow them really early; both outgrew marathons RF by about 18 months and FF before 3). If you're happy with the britax that's certainly one option. You could also look at the graco myride 65 if you are interested in a lower priced option for the secondary car (there's no safety difference).
I've not heard not to install radians on the side seats. In fact, I bought radians because they were the only taller seats to accommodate my tall children that fit in my beetle because they're low profile. The beetle is a 4 seat car with no center seat (no seatbelt and the owner's manual prohibits sharing the outboard latches to get a center install) and the radian installs beautifully in that car.
The true fit is also a great seat to look into.
Usually I suggest looking at seats with the tallest shells as most kids outgrow seats by height younger than parents expect, but since your child is teeny you will be fine with whatever seat you love!