S.P.
I have both the Evenflo Triumph and Britax Marathon. I would go with the Evenlfo Triumph. My little girl has a thick behind and legs. It has always been a tight fit locking the belts in on the Britax Marathon. It's either the seat is not deep enough, or the bottom belt (between crotch) isn't long enough??? The Triumph is comfy and safe, and an ease to use.
Keep in mind, she will eventually outgrown that Triumph though. There are other Britax models you might want to consider.