We have three Britax carseats, Roundabout,Marathon and a Husky. The Roundabout is my favorite, but I love that the Marathon has a higher weight limit, great for heavier children.
Dd (5 1/2 yrs, 53 lbs) is currently in the Husky, it's huge, heavy seat, but I love that I can keep her in a 5pt. harness this long. It can't be used on an airplane.
I think Britax seats are worth the expense.
Good luck!