If you were talking waterfowl, I'd say the SBE3 because of how easy they are to clean.
But we are talking about turkeys. The M2 has a solid, one piece receiver which is a huge benefit if you're going to drill and tap it for a red dot. It's also a proven work horse that's notorious for throwing impressive patterns.
I've got 2 M2 20 gauges. One has the standard 24" compact barrel and the other has a barrel that was cut down by comp' n choke to 19". Both guns run Indian Creek .555s with 330+ pattern densities.
I know a guy with with the SBE 3 20 ga and he would love to wrap it around a tree. Patterns are terrible and there's limited metal to actually tap into on the top of the receiver.