I have owned both. They are both great 20's. For turkey only I would go with M2, it is a little bit lighter and patterns great with TSS. If it was going to be an all around gun, I would go with SX3. It shoots a little softer and for me it functioned better with a wider range of loads. You can get both to pattern anything with a little effort. SX3 also has speed load, which I absolutely would not be without shooting ducks, crows, doves, etc.
Can not go wrong here, so congrats with whichever one you get.