There are no official guidelines for highback vs. backless booster. A booster is a booster. Use the type that fits your child best and works best with your vehicle. As a general rule, smaller children often get a better shoulder belt fit with a highback booster, and younger children can benefit from having the extra support to keep them in place when they are tired or asleep. Highback boosters often advertise side impact protection, too, but since there are no federal safety standards that apply to side impact protection we have no idea how much of that is "real" and how much of it is simply marketing.
If possible, I prefer not to use a backless booster if the child needs the little belt positioning clip on a strap in order to get a proper belt fit. And if you don't have head rests in your vehicle, you may need a booster that offers head support. Not all highback boosters offer structural head support, but obviously none of the backless boosters do.