This is just my own opinion based on my limited time with an M&P (not quite 1000 rounds). I would opt for the full size .40 caliber. It will allow you to swap barrels and use either the .357 sig or the 9mm. I have had good luck firing 9mm using the .40 cal magazines but others have had to buy a 9mm mag to make it work. Point of aim doesn't really change and factory barrels are about $65+ when they're in stock.