1x more vote for 9mm.
although I have both, I use them for different shooting purposes. I found semi-auto .22 shooting not very similar feeling to shooting 9mm and 45acp, so it seemed that it was not much of a training tool at all! (other then the basics of pointing a gun and pulling the trigger, but you can do that for free at home, its called "dry-fire" or get a pellet gun) I sold it after 2 range sessions but have since picked up a 'longer range' single shot .22 bullseye target gun which is a pretty fun challenge at more then 2x or 3x the distance and a completely different style of shooting.