If your child hasn't outgrown the 5-point harness, you should really stick with that. If you want to get a booster to use in other cars etc: a high back is generally safer than a backless, but only in so much as it has the second guide to make sure the belt is laying properly. The benefits of the back are that they 1) help the child stay sitting up straight rather than slumping to the side, and 2) keep the shoulder belt up where it belongs rather than ending up behind and arm or head or something.
With or without a back, the seatbelt is the only thing keeping the kiddo from flying through the window... the booster is to adjust his position so that the belt lays properly.
My son is 6 and 48" tall. He still rides in a 5-point harness (Graco Nautilus) in my car, but uses a booster in SO's car and for daycamp fieldtrips etc. We have a highback Graco Turbo booster as the primary booster and a backless one to leave with babysitter or whatever.
Hope this helps,
T.