Hey S.,
We used the Marathon until my 5 year old outgrew it this past spring. We had the Marathon in hubby's car and we had a Graco booster seat in my car for him. The Marathon DOES recline a bit...enough that they can tip their head back and nap. We didn't notice that much until we got the booster seat and even though the head rest has big, puffy sides, his little head still swings down in the seat, where in the Marathon it didn't.
I never thought about it, but I would guess the reason it doesn't recline more is a safety issue. I mean, it's said that adults shouldn't be riding in a car with the seat fully reclined as it's not safe. So why would a car seat designed to essentially keep your bundle of joy swathed in the equivelant of bubble wrap recline?
All my sisters-in-laws swore by the Bitax and none of us have ever had a problem. (And we love that quick-release tab feature they have for taking the seat in and out of a car!) We kept it until we felt it just wasn't comfortable for him as his shoulders were higher than the seatbelt holes in the seat(he's tall for his age).
I would highly recommend going with the Britax. Worth every extra penny in my opinion.