I need a new carseat for ds2. He is a big baby, over 20lbs at 7.5months. I want to keep him safely harnassed for as long as possible, and the britax carseats seem to have the highest weight in a five point harness (correct me if I am wrong). The marathon and decathalon have the same weight limit, so what's the difference? Which should I get for my big boy?