The best car seat is the one that fits your budget, fits your child, and fits your car. Britax is nice, but their competition has beefed up their seats in the last two years and there are a few now that I prefer over the Marathon, which was my choice seat for 8yrs.
Sunshine Kids Radian (200-$300), Evenflo Triumph Advance ($130), First years TrueFit ($250), Graco MyRide 65 ($150) are all excellent choices that will allow you to rear face your child until at least age 3 adn then forward face until the child can use a booster (at least age 5 and 40lbs). These are all within your budget. If you want a Britax, look for the Britax Roundabout 50, which is the same shell as the Marathon, but harnesses to 50lbs instead of 65 forward facing, and 35ls rear facing. Its $160 at Target and a great seat, plenty of weight for the average child.
The easiest to install would be a Marathon, however moving the car seat daily will be a pain in the rear, I would recommend getting her a good comfortable carseat for the main vehicle and then getting a Cosco Scenera or two for the other vehicle's she's in often. The Scenera is $50 at Wal Mart and it will rear face to 35lbs, forward to 50lbs. ITs a good base model car seat, safe and easy to install, and good for a second vehicle.