re: What scope should I put on a S&W M&P 15-22?

Just about any RDS or scope will do fine on a .22, since there's no recoil to disturb your zero. I have a cheap Simmons 3-9x scope on a Marlin .22 magnum, and it's one of the most accurate set-ups I have (next to my AR-10 with a Burris 4-16x scope)

I stuck an Aimpoint T-1 on my new 15-22, just because I had two NIB extras sitting in a box. The 15-22 is plenty accurate with the 2 MOA dot. I haven't found a calm day to shoot at 50 yards, but at 25 yards I'm getting 1/2-inch 5-shot groups with Mini-Mag ammo (1" groups with Wal-Mart Federal value pak ammo).

Based on what I've seen with mine, picking the right ammo will be more important than which scope or RDS you pick.

I'm betting anything in the $50 range will do fine on the 15-22, whether it's a RDS or a scope.