They are simair with the 22compact being slightly bigger, what's most important however is the manual of arms is the same more or less depending if your shield has a saftey.
I own both and use the 22 as a trainer to my m&ps and glocks frequently. They also make an excellent pistol to introduce new shooters.
I've got the Shield, 40c, and the 22c. Although they all slightly differ in size the actual feel is very similar between the three. The 22c is a very fun gun to shoot and very accurate for what it was designed for. It's also a great tool for teaching new shooters as boombotz mentioned.
I've read that others use the compact as a Shield trainer; I can see it size wise and perhaps even holster wise (it may be possible some holsters may work for both guns?).
I have a compact .22 and compact and full size 9 so the Shield is the one MP i havent added to my little collection and some day i may choose to do so. My only thoughts for using one to train on the other:
- from the one time i handled a Shield, the safety is not the same as the compact. Yes they may both have one (depending on model of Shield), but i'd think for carry use if you have a safety you better train with that safety.
- i like the idea of a compact .22 as a general trainer for new shooters as it is easy to shoot and fun as well. But if i was ready to start carrying, i'd train with the specific gun that i would carry so i was fully confident with it. And treat range time with the other guns as practice/fun time.