I've been looking at the Sig 522 myself- it's a pretty neat gun, and after handling the M&P15-22, the 522, and a .22lr chambered AK variant at the last Saxet gun show I really liked how solid the 522 felt. The M&P felt light, and the fact that it takes proprietary mags was a serious con for me.
Additionally, most everyone I've talked to has enjoyed both rifles, but found that the Sig was more reliable and ate whatever ammo was fed to it. We're talking 1 failure to fire in 300 rounds. The M&P, owned by the same person, had about 15 failures in 300 rounds.
One advantage the M&P has over the 522 is that the M&P has the same "plug-and-play" options in terms of attachments that it's big brother, the standard M&P 15 has. The Sig, unfortunately, not only ships with no irons, but also is kind of a pain to find optics for. It's just an added cost to the rifle.
Overall I decided on the Sig just because it was more accurate with fewer problems, for similar cost.