I have the Marathon for our son and I've been looking into the same thing myself for him. He still fits into it, but it sits so tall in my car that we have to be careful not to bash his head into the door frame! I've been looking at the Britax Frontier and the Graco Natalius. I really don't want to spend the $200+ on another Britax, but that one really looks like it will last us the rest of his car seat needs. The Natulis is way cheaper and has a lot more 'fancy' functions with the head support and all, but it looks like it would be a tighter fit for my son and he is already at the second slot for the harness when I put him in the floor model.
I think in the long run, I might end up buying the Nautlis and try it in car, if I don't like it in a week I can always take it back and get the Britax one.
Just do you research on car seats for weight limits and all too. I also like reading consumer reviews on websites that sell the products like Target, Walmart, and Amazon. Also look at Epinions and Viewpoints too.