I have both the M&P and the 10/22. Since I purchased the M&P, my 10/22 has been collecting dust.
My M&P has been flawless even with bulk federal ammo.
All the mods you would do to the 10/22 are factory on the M&P, so in the long run it will be cheaper.
Identical controls and ergo's as my AR-15.
My 10/22 is more accurate with a bull barrel, but not with the OEM barrel.
Accessories...most of your AR accessories will directly fit the M&P.
I believe the M&P will put huge dent in the sales of 10/22 rifles and accessories.
If you cant tell, I love my M&P. I paid $429 new at a gun show, also found extra mags for $18.99 at Gander.