I was going to use the diono but it doesnt fit in the car DH is in the steering wheel or dash and im closer than im comfortable with, so i need a good carseat that will fit in a pontiac vibe or matrix, small crossovers.
We love our Britax. It fits amazing in our new Honda Pilot, it also fit great in my old Volvo xc90 [which is a smaller SUV] and they fit in DH's little Volvo s40 car. We have the Pavilion G3 and it RF's to 40lbs and FF's to 70lbs.
The Britax has a shorter shell than the Diono, but our kids still harness to 6yo in them...so even though it doesn't convert to a booster, you'll get plenty of use out of it.