M&P15-22 allows for more parts, although the 10/22 has alot as well. I personally am saving money for the 15-22 to buy. Ive heard nothing but good reviews. I own a 10/22 on the otherhand and can testify that it is reliable, accurate and most of all, fun.
Really it depends on what you want, do you want to upgrade a pure rifle, or an AR style?
"When your goin through hell keep on going" Winston Churchill