Marathon vs. Roundabout

I just bought a Marathon for my 8 month year old and now I am wondering if I should have bought the Roundabout instead(I can still take it back). Is the advantage of the Marathon that it can hold larger kids or that you can use it longer. Since the weight capacity is higher, can I use it instead of a booster seat. I live in a state where booster seats are not enforced by law. My son
is average weight and below average in height. We do not plan to have another child until he is three or so. If we keep the Marathon we would use it until he is 31/2 - 4.

Part of my concern is the size since I am in a Saturn Sedan. The Marathon seems so large. My girlfriend assured me that it appears large because I am use to the baby carrier. I didn't notice at the store if the size of the Roundabout spacially appeared smaller than the Marathon. Does anybody know if it makes much of a diference. I won't be driving a bigger car for another year or two.