At these heights, it doesn't make as much of a difference.
Best practice/protection is a high backed booster, but in order to make a difference, it needs to be one with really good side impact protection, which means that you'll want something from a brand with very good general crash test results (Basically Cybex, Besafe, Britax, Maxi Cosi, Nuna or Recaro), ideally not a combination seat as those tend to perform a bit worse. Or an i-size booster seat, as those are crash tested for side impacts.
Also, it needs to be properly adjusted to the child so that the head support is protecting the right part of their head (check manual for guidance), they can sit back correctly with their shoulders in the seat, not pushed out of it, and the shoulder belt is correctly routed over the shoulder (not touching neck or shoulder tip, crossing middle of collar bone).
Some high backed boosters have a poor belt routing over the lap - you should check that it runs over the pelvis - and some prevent the seatbelt from retracting properly when the child moves. Backless boosters rarely have these issues. On the other hand, high backed boosters can offer more comfort and help children to sit upright without slouching (especially younger children, or on long journeys where even an older child might sleep).
One last problem with taller children is that some high backed boosters do not extend up high enough despite saying up to 150cm, and some cars have low ceilings that prevent booster expansion.
So while, yes, it's better to use a high backed booster than a backless one, that's only the case if all the above things apply! And at 140cm, if you're looking at buying a new booster so that you can extend it to full height to use for another year or two, this might be a lot of money and it might make more sense just to buy a cheaper backless one. Lastly if you have a child who feels embarrassed about using a booster seat, they may be more willing to use a backless than high back.
At 150cm your son might be able to fit into the normal seatbelt with no boost. Again you want to check whether the seatbelt fits closely to the pelvis and doesn't bother his neck. If his legs are shorter or you have deep car seats, check that he can bend his legs while sitting to the back of the seat. If he can't, then he'll slouch forward for comfort, and that brings the seatbelt up to the tummy, which is no good, so the booster is still necessary.
If your booster is an old regulation one and has a 36kg limit it's fine to ignore that limit and keep using it.