RE: thoughts on RA, MA, and boosters
You can use a five-point harness as long as your child fits in the seat; there is no law requiring you to use a booster in place of a five-point harness. NHTSA recommends using a booster seat OVER belting the child in with nothing but the seat belt, not in place of a five point harness.
In some areas, the booster to age 8 is already law - it has caused quite a stir among parents here in Illinois!
Here are some links for you to read:
http://www.cpsafety.org/articles/ConvertibleShop.aspx
http://www.cpsafety.org/articles/BoosterShop.aspx
The problem with the Roundabout and a big baby is that he will likely outgrow the Roundabout BEFORE he is ready for a booster, and you will have to spend the money on a combo seat to fill the gap:
http://www.cpsafety.org/articles/ComboShop.aspx
If you go with the Marathon, you can likely go straight to a backless booster when he outgrows the Marathon, and backless boosters are only about $20-25.
Single mom to
DS ("twice exceptional") - September 2002
DS - February 2006
DD - July 2009
DD - July 2009