Booster seat question

We have the Alpha Omega car seats that can be used as booster seats and I had
planned to use them until the girls were 80 pounds (which will probably be
college-age for Jordan) However, I recently saw some booster seats on
sale that can be used from 30-80 pounds. It is a backless booster that doesn't
do anything except raise them up enough to be able to use the regular seat
belt. My question... is this as safe as the kind we have now? When the girls
get to be 40 pounds then we could use regular seat belts with the Alpha Omegas,
but they are heavy and bulky so I thought the other kind of booster seat would
be easier to move from car to car (something we do on a regular basis). If the
other kind is not as safe then I will stick with the Alphas, since safety comes
before convenience of course.

TIA,
Andrea
twin girls-Jordan & Madison
3/22/00