it depends on the size and maturity of the child for me. My oldest outgrew the Regent at 6.5yrs... he outgrew highback boosters at 7ish... and, by 8yrs old, he was already 4'9" tall and passed the 5-step test... so since he was mature enough (keeping me from ordering an ezonpro 86-Y for him as higher backed boosters such as the Monterey and others were not out yet), we went with a no-back.
However, his brother, who had to go backless at 7, has always been a big carseat sleeper... as soon as I was able, I ordered him the Monterey after it hit the market, so he could go back in his new, taller, highback...
My daughter is now 6 (will be 7 in March). While she is mature enough, she is only in the 50th percentiles for height... and (while she used to be all torso) her torso is not tall enough (and really not broad enough) for her to fit a backless booster remotely well (the shoulderbelt doesn't even hit her shoulder... it's on her face or on her arm, depending on the backless in question)
I really don't like using backless boosters unless a child is big enough to use it without the guide clips that some have, so she won't be in a backless for some time... probably till age 8-10.
I would take a big think on this one... and ask yourself these questions.
1. how does a backless booster fit your child?? (feel free to take your child and the backless boosters you have and try him out in them to see)
2. does your child sleep in the car?
3. Is your child mature enough not to have the physical reminder of the highback to remind him not to wiggle out of position?
4. What would you prefer him to ride in??
Statistics and studies aside... I know I felt quivers of fear at every seat transition the kids made... I see you are driving an Odyssey and so I'm wondering if the issue with the back is that the seatbelts come from the top of the vehicle seat, not from the wall of the car.
Flatter belt guides would solve this problem... and the Chicco Strada Booster or even the Evenflo Big Kid would possibly work better as their guides are flatter... the BK works better in my hubby's 2007 2-door cobalt than the turbo did for just this reason.