A bit of a biased comparison. The M2 has 30 years of problem solving built into it. The thin-walled barrels are (still) prone to being dented by entry teams, which is why the manual arms now specifies not using single-point slings. The also suffered the same spring/tube issues known in the 930. Annnd, many of them can't shoot light loads.
I switched my M2s for the 930- understanding that I would not leave them stock.
So. For hard, rough use, handling a wide variety of loads- hands down the 940 upgrade (i e., 930 with OR3Gun parts and SBE piston).
For a cruiser gun, or competition, the M2.
Are you doing a shoot-till-it-stops comparison, or...?