K.L.
Britax! I can't say enough about them. I have two Regents and the kids and I love them. My son is almost 7 and my daughter is 4. My son will be in it for at least the next couple of years. He likes it better than a regular car seat.
Britax are easy to install.
The Regent is a beast but a small adult can sit in it easily. It really all boils down to which one is the best fit in your can. I would find a way to try both seats in your car.