I think I read once the reason as to why they didn't use that model was because you wouldn't be able to fire over cover. Honestly though, in the hands of what should be front-line/room clearing infantry, the second model wouldn't make much of a difference. I'd personally like to see the underslung version and be able to scream "For the Republic!".
Should have just told us to suck it up, who the hell even fires over walls anyways. 99% of infantry combat in this game is corner peaking with shotguns.