The best seat is...
The one that fits your child, fits your car, fits your budgets, and that you will use properly each and every time you put him in the car.
You'll want a seat that rear faces to 30-35 lb. The Cosco A/O and Britax RA both meet this requirement. You'll also want a 5 point harness, as they are safer than overhead bar restraints, and both seats are available with a 5-point (in fact, Britax doesn't make overhead bar seats).
As for fitting in your car, you'll have to try it out to be sure. You may need to find a technician to help you (this is recommended anyway).
To be honest, the A/O is not very highly recommended. It is not easy to use, and it is so short that it is usually outgrown before the child is anywhere near the maximum weight limit, which means you'll probably have to buy another harnessed seat in a few years. It also doesn't work very well as a booster.
The Roundabout, however, gets glowing reviews. It has a super easy to use harness adjuster. It's straps are twist resistant--more so than any other brand on the market. It has EPS foam, like is used in bike helmets, which means the child's head doesn't hit hard plastic in a crash. It has built in locking clips, which make it much easier to install in many vehicles and speed things up when you need to move it to a different car. It can also be tethered rear facing. If you can afford it, I think you'll really be happy with this seat.
If the Roundabout is too much money, there are 3 other seats you should look at. The Evenflo Triumph ($120), the Fisher Price Safe Embrace ($30-$60 at Walmart *if* you can find one) or the Century Accel ($60). The Triumph and Safe Embrace both have EPS foam, and the Safe Embrace also has built in locking clips. The quality of these seats is much better than that of the Cosco line, and I think you'll be much more pleased with them (and the added safety features) than you would with the A/O.
HTH