We love our Britax Marathon! We did not have it when Dylan was 4 months old, though. This is the one we graduated him to when he was too big for the carrier. It can go forwards or backwards, and it just felt really safe. We now use it for Carter as Dylan is in a booster.
Hopefully someone that used it at a younger age will chime in, but keep in mind that you will not have to buy another carseat after this one as I think it goes up to a significant weight!
Ann Marie