I would go with the S&W 22 to bolster transition/training with the M&P 9. I have a Ruger, S&W, Buckmark, Walther, and Sig Mosquito 22 handguns. The S&W has a much better trigger than Walther or Sig side by side...no comparison.
The S&W has controls like and feels like a centerfire handgun more than a Ruger or Buckmark. You will need to decide if you want a primary plinker/hunter or a training gun. Thw S&W will not be able to support scopes and upgrades like the Ruger and Buckmark to enhance plinking/hunting. I would choose the Ruger for a hunter and Buckmark more as a plinker. However, I have used the Buckmark successfully for hunting as the #2 hunting choice.....the S&W would trail as #3 though you may put a dot sight on the rear and enhance it. The dot sight would also enahance personal defense use/training especially if your centerfire defense weapon has a dot sight.
If you want something to train with and to save money with when training and that will absolutely transition to the M&P 9 or other centefire handguns (I carry a Ruger SR9 and M&P 9 or 45) then it must be the S&W M&P 22.
Just my 2 cents. Can't even by a Fireball for 2 cents these days.....