The M&P22's slide is aluminum. I own one and the manufacturers website lists it as aluminum. Reader beware on many internet forums. Its not potmetal.
I really like my M&P22. Its for all intents and purposes the same size as my FS M&P 9mm and fits in my kydex holsters. The trigger is better then the FS guns from the factory as well. It has been extremely reliable using a wide variety of ammunition. Its also very accurate. Its been a good purchase so far. I've also heard very few issues with them from others, even ones with higher round counts.
The scaled down .22lr 1911's seem to be pretty decent guns. I hear more about reliability issues with them then the M&P22's but for the most part, the ones I've seen run pretty good. The M&P would be a better training platform for someone looking to move onto a striker fired type gun like an M&P or Glock. I'd give a slight nod to the M&P22 but it comes down to what platform you want.