My 20 month old son is 29 lbs and 33 inches tall. He has a very long torso and his shoulders just now at the level of the highest strap position in his Century Accel. I have not been very happy with the Accel and wish to stay away from Century products. It is almost impossible to keep the tension of the straps even. Our other seat is the Roundabout and I love it. I am leaning towards purchasing the Marathon. However, I am wondering if a combo seat would be more economical since it could be used as a booster later. Any suggestions?