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.