I absolutely love and highly reccomend the Britax Marathon. Pricey, yes, but rated as one of the best on the market when I was shopping for carseats. Both my 1.5 year old and my 3.5 year old ride in them comfortably. It holds up to 65 lbs, so my 3.5 daughter who weighs about 35 lbs, still has plenty of time to fit in it, which makes it worth the investment. I have many friends who have gone through 3 and 4 carseats per child, for lack of fit/comfort. My kids have no problem with the Marathon, it is cushy and comfortable enough to sleep in, which they seem to do as soon as they get into them.
I do have a less expensive graco car seat in the second car, but it seems so much less comofortable by comparison and doesn't fit great to my child (there is so much empty space around them and it doesn't have a lot of padding). We use it mainly for drop-off/pick-up for pre-school.
I feel my babies are so much more safe and secure in the Marathon. They are so well made and I just feel that when it comes to my precious cargo, I want them to have the best protection and I wouldn't get on the highway with them in any other seat. They have been worth every penny.
I bought mine at Goores and they installed them for me, which was great.