I've installed both with LATCH, but not with a seatbelt--That would probably make a difference since the Marathon has a seatbelt lock-off, and the EFTA doesn't. With seatbelt installations, some people have trouble with the seat leaning to one side because of the upward lift of the shoulder belt when used rear-facing. That issue can usually be resolved by using the lock-off on the Marathon, or a locking clip on the EFTA.
With LATCH, both were easy for me to get tight rear-facing and forward facing. The only thing that bothered me about the EFTA is that I wish they'd made the LATCH strap a little longer, and the loop in the end of it a little bigger to make it easier to grip while tightening. Really, I think they're both great seats though. I know several people who use them, and they both seem to fit well in different kinds of vehicles, and fit kids well. I would just try the EFTA in your vehicle to make sure that you are still able to turn the knobs when installed (sometimes they squish too far into the vehicle seatback upholstry, depending on the vehicle). Good luck!!