Both are excellent firearms. I would highly recommend finding a rental range that has one or both and try shooting them. I cannot say which is better, but I can say that everything the XDM is, the M&P is just as good at $100 or so less.
Here's a link to the 357 SIG conversion for your XDM.
"The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government." - Thomas Jefferson