Comment about using the Britax Marathon:
It is very heavy and cumbersome. Buying the Gogo Kidz wheels helps but be aware that another problem might occur.
When we flew a few years ago to Berlin from EWR with CO (757) we had the same problems as described by other comments - very hard to fit the seatbelt, very limited leg space left for a 17 month old in a coach seat ! since the britax is big our daughter was packed tight in the seat.

On the way back the car seat would not fit through the x-ray machine in Tegel and German security would not let us bring it on board (they refused to check it by hand instead). After a few tries I took off the gogo wheels and shoving it in, I managed to get it into the machine (I thought it will be a Pooh bear situation with our seat stuck inside the machine.....) thus we managed to get it to the plane.

On a different trip we checked it at the gate and asked to get it back at the ramp after landing - well, instead they sent it to the carousel and this time it did get jammed and no one could get their bags for more than half an hour until a mechanic came to clear the blockage