I'd recommend dropping the extra bucks for the M&P. I own both, and though the Sigma goes bang every time, that trigger is not conductive to good shooting habits if you're just starting out. That and the grip on the M&P is so much more comfortable. I've run into numerous people at the range and gun shops who had bought a Sigma and were looking to get rid of it, specificly because of the trigger. They pick up the M&P, smile, and then dry fire it. A difference like night and day.
And after you buy the aftermarket springs, you're near the M&P price anyway.
For a cheap, reliable, disposable gun, the Sigma will do the job, and it doesn't feel all that bad. But if you're serious about using your purchase as a carry piece, give a look to the M&P before dropping the dime. Some will champion the heavy Sigma trigger as an extra bit of safety, but one striker-fired gun is the same as another. Keep your booger-hook off the bang-switch, keep your shirt out of the way when holstering, get a quality holster, and there won't be a problem.