L.S.
I LOVE. Love. Love my Britax Frontier. My son is 3, and is 43" tall, and 42lbs, and he is SMALL in this seat, and LOVES it. It has an adjustable high back, and also has 5 pt harness for up to 60lbs I think. The seat goes up to 80lbs, but then is used as a booster seat with the car belt. Unvlievable seat, well worth the money!! I got mine off Target.com, and got free shipping in a few days. It is priceless for big boys. My son sleeps in this seat in a heartbeat, and always comments on how much he likes it. Also, kiddos like the fold out cup holders, and moving arm rests..
Good luck!