We have both and LOVE both. My oldest daughter (3 1/2) has always been my "bigger baby" (95th% for weight & height)...we put her in the Marathon at 8 1/2 months old...and she's still in it at 3 1/2 years old, with still lots of room for her to use for a long time to come. My youngest daughter (21 months) is our "small baby" (10th% for weight & height) is in the Roundabout...with still lots of room for her to grow. SO....although I LOVE both carseats, based on my experience with my oldest daughter being (like your daughter) "big for her age" (she weighed 26lbs. at 1 year), I would go with the Marathon, if I were you. A little bit more $ than the Roundabout, but you will get a lot longer use out of it with her being a tall/big girl....and it will hold her until she is ready for a booster. Again my daughter is 3 1/2 and I have no intentions on moving her out of the Marathon until she reaches the max. weight limit for it...due to safety reasons and the 5 point harness being safer than the booster seats.
Hope this helps. Godd Luck!