Another vote for the Britax Regent. I bought it for my son when he was 5 and he is 8 and just came out of it. I'm SO GLAD he had that protection. I would NOT get a flimsy Graco high back booster (unless I had no other options). You can feel the difference in EPS foam protection, though to be fair the newer ones seem better than the ones out even a year ago.
http://www.youtube.com/watch?v=azgBhZfcqaQ
While it's true at 5 he "could" use a booster till he is 6 years old and 50 lbs, that is not necessarily the safest.
There is a difference:
http://www.youtube.com/watch?v=V2kO8AxKbrM&feature=re...
Different states have different laws. California says at least up to 6 years and 60 lbs. Utah & Massachussettes just put new laws on the books that say up to 8 years old and (or?) 80lbs.
Canada has different laws, but they are also saying minimum 4'9" tall OR 80 lbs OR 8-9 years old.
My son is 4' 4" tall and he is still in a booster. He doesn't like it. He wishes he could be out of it because some friends asked him "why are you still in a baby seat"? The Regent is NOT a baby seat. People just don't know.
He said he did love the comfort and he would still be in it his friends stopped bringing it up.