If possible, get something otherwise identical to your 9mm, so you are not changing your grip and sighting for a totally different platform. If you have an M&P 9mm now, an M&P 22 makes sense. Glock 19, get a Glock 44. Again, if possible.
Tozan brings up the 22A.. I have one and shoot it a fair bit, but it is pretty different than all my 9mm pistols, so I'm not sure training with it helps me much when it comes to the 9mms. It's more of a target pistol. It is accurate though, and has a nice rail for adding a pistol scope to it, which helps tremendously in the accuracy department.