Regardless of bells and whistles, competition leads to better products. The original S&W Sigma was fair at best, Bergeron who came to S&W from Colt was given Carte Blanche to design a completely new pistol for S&W. The M+P was the result.
See American Hangunner Jan/Feb 2010, pg 28. Massad Ayoob goes through the reasoning why more LEO agencies national and international, are getting M+P's.
Glock, is trying to keep up. They always made a great pistol, now its down to features and creature comforts, that WE the buyers want. Thankfully, I have large hands, because my G21 grip is uber big.. My M+P has 3 options for the back strap. It's just the nature of the beast.
Just like the Brand New Mustang vs. Brand New Camaro debate, they are both nice cars, just one of the great aspects of capitalism and marketing.