Both are good guns, excellent for the money, it depends on what feels better in your hand. Glocks are based on the luger grip angle, M&P's more the 1911. While I appreciate Glocks for the fine firearms they are, I despise the grip angle, therefore M&P's are my choice.

That said, if you like them equally, Glock parts are cheaper and offer more drop in options. Go with Glock.