Good points, the DBSG is one of my favs and ammo is way more affordable than the SG. I use Crafter 3 in this loadout to get plenty of ammo.
SG vs DBSG, well that's obvious, the SG will get you each time, has longer range and acts like a semi-auto SG.
The Shorty has the benefit of being silenced, also with Maximum upgrades, I have downed opponents with one shot at close range. The animation shows them flying backwards, no armor and probably not 100% health, but the upgrades do wonders with the Shorty.