I own several 1911's, including a ParaOrdnance 'fat body' which I built from the frame up. I also own the close cousin of the M9, the Beretta Model 96 in .40 S and W. Both guns are good, solid tools which will perform as intended. But if I had to choose, I would choose the 1911.
The Beretta can be a little tempermental. I like to shoot the little 155 grain lead semiwadcutters in the Beretta and with that bullet jamming was a real problem due to the feeding characteristics of the bullet configuration. I solved that problem by CAREFULLY opening up the throat just enough to feed reliably, but not so much as to cause the loaded round to be unsupported by the chamber. The .40 is a high-pressure round, and over-zealous chamber throating is not recommended for any gun in that caliber. Even after that minor adjustment, the gun would still jam after about 50-100 rounds. The problem is that when the breech face gets dirty from powder residue, the case head of the next round will not slide nicely in place with the rim neatly behind the extractor. I sort of solved this by carefully polishing the breech face. Its good for about 100 - 150 rounds before I have to clean the breech face.
The 1911 , on the other hand, will shoot 500 - 600 rounds without a good cleaning if the feed ramp and breech face are polished. The one thing I cannot get used to about the Berreta in a combat situation is the transition between the first and second shot after the holster draw; that's because the first shot is double-action and the follow-up shots are single action. The transition between first and second shots completely changes the ergonomics of the grip and the position of the trigger finger. Also, since the Beretta has a two-stage trigger pull in the single action mode, this transition between first and second shot is diffcult to master.
Unless our troops are really well-trained in the use of this unit, then I hope that when they have to use it in combat, that their intended targets are very close.
Other than that, I think I'd rather have 7 rounds of 230 grain .45 ball and three or four extra mags, than 15 rounds of 9 mm, although either caliber would be unpleasent on the receiving end....
My problem with the Berettas, beyond the trigger pull/action issues, is that Ive never forgiven them for that little slide problem in the '80s. I like to think that my dollars go to companies that put me first in their business equation, a company thats has such poor quality control certainly didnt have my best interest in mind. There will likely never be a Baretta or S&W (yes, Im still boycotting them) in this house.