The team members of a 10thgroup team I knew were all carrying M9's with Brigadier slides and night sights.
The guys from fifth group were a mixed bag of M9's and 1911's ranging from stock 1911A1's from WWII/Korea/vietnam to moderately customized guns.
One soldier was carrying a 1911A1 with Ed Brown drop in match barrel, grip safety, S&A mag well, Ed Brown Ambi-safety and 10rd stainless magazines. They got replacement sight sets from somewhere and he was annoyed that the front was tritium but the rear not.
he mentioned their armorer would modify his 1911 for parts and beer.
They get to choose which handgun they want to carry but it seems most carry M9's. That's probably a combination of lighter wieght, greater reliability, better accuracy, lighter recoil, and higher magazine capacity. The SF magazines are Beretta factory, not checkmate mags like half the regular army mags.
I think the M9 is the right choice given that the 1911's they have access to are poorly fitted and most aren't very accurate or reliable. Ammunition is more availible in 9mm as well, so they get more range time with the M9s. [and I'm a 1911 fan so that isn't a biased opinion.]