I went with the Graco Nautilus (actually one for each car) when my son outgrew his original seat. It gets excellent ratings, has a steel frame, works with a 5 point harness up to 65 pounds and then as either a high back or low back booster after that. It lists for $190 but I got mine at Cosco for about $137 which I think you can also find on Amazon and other sites.
I looked at the Britax and the Graco and couldn't see spending the additional $$ for what didn't look any better to me in the safety ratings. Very happy with the Nautilus.
The down side is it is HEAVY (steel frame) - 28 lbs - so we do not use it for airplane travel - I bought a low back booster for when we fly out of town.