Ok, we've established that both the .308 and the 6.5G are good rounds at distance. I use the term 'distance' as a generic term, generally between, let's call it, 400-600 yards. Someone will yell at me for that, and I'm ok with that.
Perhaps it could come down to price, but if we go down that route, we have the following conundrum:
6.5G uses AR-15 lowers and components. AR-15 parts are generally more common and lower-priced in comparison with .308 parts. But...
6.5G barrels aren't exactly common, and only a few manufacturers are producing them, so maybe the cost advantage of AR-15 parts is offset by the barrel price come build time.
Oh, and Tirod...bring me the 14.5" .50 BMG 'carbine'. I'll throw a red-dot on it and we'll hit the range. I'm pretty sure a 30-round mag would be about five feet long, but I'll deal.