If you have a tall child and are interested in extended rear facing you should cross the britax off your list. They have notoriously short shells, so they are outgrown both RF and FF long before most competitors. My girls had a classic marathon (a little taller than the current generations) and did not make it to 2 RF or 3 at all in it because of the height limits.
I have radians and have never had trouble getting them installed (in a variety of different cars too). I'm a huge fan. The other seat you can look into if cost isn't a huge concern is a clek foonf, which is currently the tallest seat on the market and, like the radian, very narrow. It also has different technology for a forward facing child that looks like it would be much better than a standard carseat as far as keeping the spine in alignment in an accident.