Is there an accuracy/reliability difference between the Complete AR.22 (like the S&W) vs the CMMG upper?
In my experience, the CMMG uppers are very reliable with a wide variety of .22LR ammo. Here is a link to my experience with my CMMG WASP upper:
I can't comment on the S&W M&P15-22, as I don't have extensive experience with them, but the concensus is that they are quite reliable as well.
I believe either option is a good choice, but as mentioned above, I prefer the parts interchangeability, feel, and construction of the CMMG offering over the S&W. I lilke the fact that the CMMG is a real AR-15 (chambered in .22LR), as opposed to the plastic proprietary construction of the S&W. I can confirm that the CMMG uppers are very accurate out-of-the box.
The CMMG upper didn't used to actuate the bolt hold open. You had to get an add on kit and use special magazines.
I don't know if that is still the case or not.
The CMMG upper will hold the bolt open, but only if you install a Bolt Hold-Open Adapter (BHOA) in the lower (it's a drop-in piece). You will need to use either CMMG magazines or the Black Dog magazines (my preference) with hold-open compatible followers.