We decided by age 4 to move up to a booster seat, however we bought the ones that have the back that detaches when needed. We believe that the headrest provides much needed support in the event of an accident and also works great on long road trips to let the kids nap and rest their heads.
We loved the Marathon/Britax seat and used it since getting out of the infant carrier. It was great until about age 3 1/2, but then their legs dangle, the seat is bucket style, but as they get bigger it's very restrictive for them to move their arms and shoulders. They also complained that the 5 point harness "clicking" in the crotch area bothered them too.
So by age 4 we moved to a Graco Turbo booster. ALL of our friends have the same seat too and we have extras for carpooling from scouting meetings, sports, etc. Most people do not have/use the back support and it can be purchased without (base only). We use the back support for ages 4 and 5 and by age 6 remove it and only use it for the long car rides.
It's a great, safe, easy to use car seat.
If you are going to use the Britax, I'd be ready for the complaints of being squished in that too. Plus I think boys have more issues with the crotch snapping of a 5 point harness.
Best wishes, I hope this helps.
http://www.google.com/products/catalog?q=graco+booster&am...#