My wife would be much better at answering this than me, but I'll give it a shot.

Be careful with the Britax Marathon - it may be too wide to fit properly in an airplane seat. Based upon recommendations from others, we bought an inexpensive 2nd/travel seat from WalMart (Torino?). We've got the Britax as #1 and it really isn't going to work well if you're doing a lot of travelling. Not only is it big, but its also pretty damn heavy to carry through the airport (with a baby, stroller, diaper bag, etc.).

There are message boards at http://www.tcoyf.com and I know this topic is discussed in detail there, I'm trying to find the relevant topics and will edit this post to add them if I find them.