If you are concerned about price, the Fisher Price Safe Voyage Deluxe runs about $100 and harnesses to 55lbs, the Cosco Apex runs about $150, and harnesses to 65lbs.
HOWEVER since yo usaid your son is tall, try this. Have him sit on the floor with his butt against the wall, then measure the distance from his seated rear end to the tops of his shoulders. THAT is the strap height/harness height. My guess is the carseat he outgrew only had a 14 or 15 inch top slot, and alot of other seats have a 17 or even 19 inch top slot. Thats a big difference.
The Safe Voyage Delux has a 16 1/2 inch top slot, the Cosco Apex has a 17 1/2 inch top slot, and plus they harness to a heigher weight so he'll be able to use it well into Kindergarten. THe Britax seats range from 17 to 19 inch top slots, and again the heigher harness weight allows for longer use.
I guess itsn ot a question of money for me, I want what is the safest and what can be used the longest. If you pay $250 for a seat and it lasts for 3-4 years, and you can get another 2-3 years out if it with another child, then in my opinion its worth it. Remember seats expire after 6 years.
www.albeebaby.com or www.specialtybaby.com has the best prices on carseats nad Britax, and offer free shipping.