C.M.
I would recommend Britax because all of their carseats are plane-approved. They are also very comfortable--we had to return a Cosco Alpha Omega Elite because my daughter cried hysterically the few times we put her in it. She loves her Britax, though. We have the Marathon model because my daughter is tall and on the slim side, but the Roundabout is smaller and also works for her. I have a friend who has the Marathon in her SUV and the Roundabout in her 2-door Chevy Cavalier, which is a small car. She says it fits great. Either way, though, you should know that you can go to Babies R Us and try out the carseats in your car without buying them --you can just talk to them in the front.
Also, Fisher-Price also has a model, the Safe Voyage, that is actually made by Britax but is much cheaper.
Finally, I wanted to mention that Albee Baby (http://www.albeebaby.com/) seems to have the best prices on Britax and other gear. They also usually offer free shipping.