Umm... Well let me try to explain. The P229 is really chubby so if you have stubby hands, I don't think that is the best choice. I personally think the Sig feels just right if you have larger hands... Like, the web of you hand fits really snug to the top of the frame for good control over it. But if your fingers are short, its a little difficult to reach the double action pull without moving your grip but if you don't care and shoot single action all day its okay. But its manageable. And I personally think it is better for ambidextrous use if you wanted to do that.
While the M&P feels good and gets the job done. Like you said, it has the adjustable back straps too. It just works for any size hand
When I have the two side-by-side, the feel of a metal trigger and frame just feels different. I like it a little better tbh. I think your best bet is going to a gun shop for yourself. Let me know if that makes sense if not, I will take a photo