We have two boosters in our car, a high back and a backless, because we carpool with a friend and need a seat for her (our town doesn't have busing). Around town I'm fine with Julia riding in either. For long drives or if it's late at night she rides in the high back because it supports her enough to keep her upright while sleeping where when she has fallen asleep (exactly once) in a backless she slumped right out of the shoulder belt.
As long as a child is sitting correctly there's no significant safety difference between a high back and backless, but the high back provides better support, so it's more likely they'll stay in position in that for long rides or while sleeping.
Julia seems to be one of the few in her K class still in a high back, but then Emma is one of the few in her 3 year old preschool class still in a properly fitting harnessed seat (the few still harnessed are mostly in long outgrown seats) and many are in boosters, some backless. So I don't follow what everyone else does around me.