First off, I follow a golden rule with shotguns: NEVER shoot one unless you can hit AT LEAST 2 enemies/1 BIG enemy or it's not cost-effective!
For general purposes, I use the Shotgun + AA12, saving the AA-12's EXTREMELY limited ammo for desperate situations, and the Shotgun for everything else. Works well enough, as the Shotgun handles all the small stuff and can be freely used with few ammo worries. However, even with the auto-shotty, it lacks the sudden "Drop them!" factor and due to this, many enemies can stay alive JUST long enough to hit you. As an added bonus, you can add a Handcannon for single enemies (CRAWLERS)
If I have a team that can be counted on to back me up (Keep single enemies off of me so I don't waste ammo), I go for the FIREPOWER PACKAGE of the AA-12 and Hunting Shotgun. Blast the hunting shotty into crowds, use the AA-12 in semi-auto occasionally on the small pockets that aren't worthy of an HS blast.
Against the big stuff, use the AA-12 until they're near death (Harder to tell with FPs, but with Scrakes, it's when they charge) and deal a finishing blow with the Hunting Shotgun. It will DROP any raged Scrake (Who, by the time you have it out, are nearing point-blank and prime targets for the full HS power!) immediately with a solid hit.
Ammo and cost are the only issues. It's not exactly a cheap option to use, but the HS 'Final blows' will pretty much ensure you get some money from the big ones to keep you going, not to mention how much you can get from emptying 2 HS barrels into a packed hallway!