M&P, particularly if you have any question whether you like the feel of the Glocks. I haven't shot a Glock. Bash me if you want for not liking a gun because it shoots nice, but doesn't feel right in my hand. With the selection out there, no one needs to own a gun that doesn't fit their hand. Most of the ones that have been listed here have little to no problems with their current production models. The only difference between a Glock and everything else, is that it's been out longer and has cheaper replacement parts.
I have fired the XD-9c, and it is a nice shooter. I personally didn't like how high it felt compared to my M&P40. Your results may vary.
The M&P is nothing like a Sigma. I'm not saying the Sigma isn't a good gun. I have one, and with the exception of cheap ammo, it's never failed me. I think Smith could have done better to give it a softer, more grippy, grip, and to lighten the trigger pull. Also, the unsupported chamber makes reloading dicey.
The M&P is steps above a Sigma, there's no unsupported chamber issues, you can shoot lead all day long with no worries(compare to H&K poly-barrels), the ergos get wide-spread kudos, the trigger is just about right after a short (150-200rounds)break in, with option to lighten if you want to send it out. Should you ever need Smith's service, it's top notch. And the price is better than most of it's equals.
Try them all out and pick what fits your needs. Life's too short to have a gun you don't like.