I've got one of each and would say it depends.
I reload for the 9mm and have some rather hot +P+ rounds that launch the 115 grain bullet down range with some velocity. Those things snap the wrist quite well and cause lots of muzzle flip. The nice wide body of the double stack magazine and the rubber grips of the Beretta do a good job of softening the snap.
I don't reload for the .45 yet as I'm still shooting up factory ammo to get the brass. Shooting 230 grain FMJ rounds the single stack .45 wearing wooden grips doesn't snap me with the same velocity as the 9mm but does push back pretty hard.
What I'll have to try is shooting some normal factory loaded 147 grain 9mm vs. some 230 grain .45. I would guess that the nine snaps where the .45 pushes more.