Given the environment you're considering, you're gonna want a hard chrome finish on the bore & chamber. You're not gonna get that with the Sport. There's also a few other details like no heat shield in the hand guard. I used to swap electrons with a guy who at the time was CO of the USAMTU. He stated that the non chrome bore treatments didn't stand up to rapid fire like hard chrome does.

The folks who make barrels/gas blocks/bolt carriers (very limited # on the last) likely don't waste time doing different exterior finishes, they're probably all mil-spec or very close to it. The anodizing on the aluminum parts is gonna be done by the manufacturer. Remember many of those makers are also selling either parts or complete rifles to the DOD or other government agencies. (But beware the low bidder!)

Given personal experience with both manufacturers, I'd go S&W, but I'd pony up the extra $ to get the M&P15 rather than the sport. Or, go Rock River.