Trust me - you want the P99.
There are some Sigma fans - but honestly - the Sigma isn't even in the same class as the other 2. The Sigma will be reliable, but people always get sucked into Sigmas because of the price. The trigger is VERY heavy. It may not seem too bad trying it in the store - but once U hit the range and try to hit the bullseye, you will soon find out how heavy it is.
Over the years, I have seen dozens of newbies go buy a Sigma, and then their next question is - "How do I lighten the trigger pull." Now, a Sigma might make an ok truck gun - that's just an occasional shooter. But if you plan to hit the range a lot and you want to hit the center of the targets, you will be shooting a long time before you can do it with the trigger...
As for the M&P and P99 - everyone here knows I am a P99 fanatic - IF you get the A/S trigger. I do not like the QA trigger. The P99 A/S is my all time fav handgun, and I even shoot it better than I do 1911s. Plus, I've yet to find a more ergonomic gun. And, what is also cool - you need not go to a smith to adjust your sights. You can adjust them and/or replace them yourself (with night sights) No need to go to a gun smith for that.
I have rented the M&P in both 45 and 9mm. I just don't care for it that much. The trigger doesn't seem to have a place that's easy to tell when its going to break. Once you shoot the P99 about 200 times, the trigger smooths out and is not as gritty as it feels in the shop. The SA pull of the P99 A/S is so sweet.
I've had to take the CHL class (here in TX) 4 times (1 to get it, and 3 renewal classes). There is a shooting test each time. My 2nd and 3rd time, I used a 1911, and I got somewhere between 242 and 245 of 250. With my P99 A/S last summer, I got 250/250, and I hadn't shot the gun ina several months at that time, as I had been practicing with a new USPc.
Get the P99!