The Sig M17 and Sig M18 are a version of the Sig P320 and were adopted as the standard issue sidearm for the U.S. Military. The Glock 19x came in a very close second in the modular handgun program. Comparing the Glock 19x to the Sig P320 can give us a better understanding of why the military decided to go with Sig.
The Glock 19x that was released to the public gun market does not have the manual safety like the one send for the military trials. Many gun collectors would prefer the manual safety so the gun would be more closely aligned to the submitted design.
The Sig P320 M17 and M18 are virtually identical to those submitted to the military except for some of the coloring on smaller parts.
One of the reasons for the military choosing the Sig is due to the ease of replacing the grip module to fit the shooter better and in the event of it being damaged. Damage to the Glock frame would result in the entire gun needing to be replaced.
The real winner in the comparison between the two is the gun enthusiast. We have two more very good firearms to choose from.