K.T.
Britax is great!!! I got the Roundabout and that fit my son till he was about 2 years and 4 months (he's tall and it only goeas to 40lbs and 40") so I totally agree with Andrea and say get a Britax but get the Boulevard or Marathon to last you longer. He is now in Sunshine Kids that is a 5point harness/booster seat.
The biggest factor, keep them in a 5 point harness till as long as you can, Britax models offer the best.