I would never put a child in a booster seat using only a lap belt. The occupant's body will jackknife over the lap belt at the waist in a collision potentially causing severe abdominal injuries, fractures of the lumbar spine, and serious head and facial injuries.
Which type of booster you get depends mostly on your vehicle. If your vehicle has low seat backs or does not have any head restraints, you will need a high back booster to provide whiplash protection for your child. If your vehicle does have head support, a backless booster is fine. However, some younger children prefer a high back style, as the side "wings" allow for more sleep support. Also, some older children prefer a backless style that does not "show" as much and cause teasing from peers. You must be absolutely sure that your child has adequate head support before using a backless style. Remember, safety before popularity.
Booster seats are designed to raise your child so that the lap and shoulder seat belts fit properly. This means the lap belt lies low across your child's upper thighs and the shoulder belt crosses the middle of your child's chest and shoulder. Correct belt fit helps protect the stomach, spine, and head from injury in a crash. Both high-back and backless booster seats are available. They do not come with harness straps but are used with the lap and shoulder seat belts in your vehicle, the same way an adult rides. Booster seats should be used until your child can correctly fit in lap and shoulder seat belts (see "Seat belts" below).
Your child should stay in a car safety seat with a harness as long as possible before switching to a booster seat. You can tell when your child is ready for a booster seat when one of the following is true:
* She reaches the top weight or height allowed for her seat with a harness. (These limits are listed on the seat and are also included in the instruction booklet.)
* Her shoulders are above the harness slots.
* Her ears have reached the top of the seat.
This site is useful for comparing different car seat brands and models. http://www.nhtsa.gov/CPS/CSSRating/Index.cfm#Booster