The safer choice is to keep your child in a HWH (high-weight harness) seat as long as possible before going to a booster. Two choices are the Britax Regent and the Sunshine Kids Radian80. A harness seat is safer than a booster seat. A forward-facing harness seat is outgrown when 1) the child exceeds the manufacturers maximum weight for the seat, or 2) the childs shoulders are higher than the top harness slot of the seat. Do listen to Jamie C's advice about looking at the Kyle David Miller web site.
Boosters help make the vehicle seatbelt system (designed for an adult) fit a larger child. If you decide to go the booster route, then choose a booster that your child fits into well. The seat belt should lay flat on the child's thighs, not leaning against the stomach. This is critical, since the seat belt must lay against bone (hips), not soft tissue (stomach). A shoulder harness MUST be used with a booster and should lay between the shoulder and the neck.
High-back or no-back booster? High-back boosters (HBB) have the advantage of giving the child a place to put their head if they fall asleep in the car. Some HBB have additional side crash protection. However, be aware that some HBB require that a vehicle head restraint (headrest) exist behind the back of the HBB booster. It's a good idea to read the owners' manual online before purchase. Low-back boosters are small, very inexpensive, and transport easily from car to car.
Finally, your child will no longer need a booster seat when she can pass the "5-step test" (see http://www.carseat.org/Boosters/630.htm). Note: she may no longer need a booster in YOUR car, but she may still need one in your neightbors car-pool van. Do the 5-step test in every different vehicle she rides in. The laws are minimums only (in CA, 6yrs or 60 lbs) and do not mean your child is safe without a booster at 6/60.
Let me know if you have any questions about car seats.
Because I care,
-- M. Cohen