I would also suggest looking into a britax roundabout, it would be way cheaper than a bigger car, and that way you could choose the car that you like best and best fits your lifestyle rather than just your car seat.
The roundabout is a britax, and still very safe and can do extended rear-facing, but it has a smaller shell, and I can fit it easily in my Honda Accord rear facing and still have plenty of room for the passenger.
The thing about the marathon vs the roundabout is that the marathon is outgrown rear facing when the child shoulder height hits 16 in, and in the roundabout it is 15.75. That is not very much difference, the big different between the two is weight, and most kids will outgrow rear facing in height long before weight.
The roundabout is a great alternative to the marathon, fits better in cars, and is cheaper too!