With belt-positioning boosters, one of the key factors is, not surprisingly, how well does it position the lap and shoulder belt on your child. The 3 in 1 seats like the Cosco/Eddie Bauer Alpha Omega tend to do a very poor job of positioning the seat belt on the average child (tends to place the lap belt too high up, over the soft abdominal tissue, rather than low across the tops of the child's thighs - please see the article and photos at this link:
Is your 6yo using this seat as a harnessed seat or as a belt-positioning booster? If she's using it as a booster (lap/shoulder belt without the 5 point harness) please check how it positions the seat belt on her. Chances are that she would be better off with a different booster. If she's still using it as a 5-point harness seat, please check that her shoulders are still at or below the harness slots as many of the 3 in 1 seats have relatively short top harness slots that are frequently outgrown before the child has the size and maturity for a belt-positioning booster (and in many cases the top slot of the seat is to set the seat up as a booster and is not for the 5 pt harness - please check your manual about this).
Depending on your budget - if you wish to keep her in a harnessed seat (Carseat dictionary moment - techically, a "Booster Seat" refers to a belt-positioning booster, which has no 5-point harness and only positions the vehicle's seat belt on the child; a forward facing seat that can be used as a harnessed seat or a belt-positioning booster is called a "Combination Seat" - the confusing thing is that car seat manufacturers themselves frequently call these seats "booster seats"!), some good "Combination seats" recommended by certified child-passenger safety techs (CPSTs) for how well they work both as harnessed seats *and* as belt-positioning boosters are, in order from least to most expensive, the Evenflo Maestro (about $80 retail), Graco Nautilus (about $160 retail), and Britax Frontier85 (about $240 retail).
If you feel she has the size and maturity to use a belt positioning booster, some that are frequently recommended by CPSTs for positioning the belt well on smaller booster riders include the highback Graco Turbo Booster (I've seen them recently at Costco for around $30), the Britax ParkwaySG (has a "SecureGuard" clip that attaches to the lap portion of the belt to prevent smaller booster riders from "submarining" under the lap belt in an accident).
(and BTW my 9YO twin DD's still use boosters and will continue to do so until they pass the "5 step test" described here: http://carseatblog.com/3966/the-5-step-test/ ). Yeah, I'm one of *those* kinds of moms ;-).