All guns you mentioned are good, all also have thier own issues: The Beretta is a good gun, but its on the large side, is complex in design and is based on 1970's technology. The beretta's trigger is improved by the addition of a D main spring, this helps to smooth out and lower the double action trigger pull. Unknown to most is that Beretta offers a short trigger option for the 92 series similar to the short trigger option Sig Offers. If you get a Beretta make sure you update the locking block to the newest design, it will be radiused on the ends, Also beretta recomends you scrape the carbon from you extractor every 1500 rounds.
The sig is solid but suffers from a high bore axis and muzzle flip is more noticeable in the Sig. I dont trust current Sigs as I feel the quality is suspect. Older SIgs have a much better build quality. I know of one major local city Dept that has got rid of 2 year old Sig P-229 Daks becuse of reliability problems. The new Sig does not build guns to the same quality as the Sig of Old.
THe M&P is a modern design, has a low bore axis and great ergonomics. The triggers are Ok on Free state models and suck on ther MA versions. A trigger Job by Derr or Apex trigger parts solves this issue. From an armorers stand point both the Sig and Beretta Suck compared to the M&P, both utilize lots of small parts and roll pins etc. Other then the sear block assembly the M&P is simple to disassemble. In regards to the sear Block S&W recomends you just replace the whole assembly instead of taking it apart. The newest version of the M&P with the new tacticle reset trigger is supposed to be "More Glock Like then a Glock" I imagine once these come out the gun will sell even better. The M&P is not the Piece of crap people make it out to be. People in the know like Kyle lamb and Pat Rogers speak highly of them and carry them. In the end, get which ever one fits your hand the best, none of them are perfect and all have thier issues. YMMV