I have never used the Sigma myself, though overall I have heard far more complaints than complements about them.
I have shot the SW99, and have mixed feelings about it. I agree with the statement posted earlier that the SW99 has too many firing modes, controls, etc. There are at least THREE positions for the trigger--who needs to learn 3 different trigger pulls on one gun?
Of the three pistols you asked about, the M&P is the newest design, and we presume benefits from whatever S&W learned making the other two and receiving feedback from end users. It has the best trigger of them all, IMO, and the grip ergonomics are excellent (perhaps matched by the SW99).
With the current $50 + 2 mags deal (on a pistol that is already a good value), I don't think you can go wrong with the M&P.