With the shotgun you get superior versatility. Upland game shells are cheap so you can really can get used to it. If it ever corroded on the barrel's interior (harsh climate exposure?) your performance won't suffer as dramatically. Then again, are SKS barrels chrome lined (chrome lined barrels are very resiliant)?. Shotguns have a much wider selection of uses with various shot sizes and slugs, in fact, you could even get extra barrels to further this goal. Mossberg used to sell a kit that came with an 18" home defense barrel and a 26" adjustable choked barrel (which allows you to control the pattern/spread of your pellets). Besides, police and military worldwide use pump shotguns everyday...who regularly issues the SKS? Of course, you could get both for under $300 if you hunted around (perhaps not the 590 though). Just think, all that range and power for half the price of your pistol.
FWIW, I love my Springfield 1911, but why would a long gun take 2nd place to a pistol? I think most new comers to guns favor pistols (I did), but in time you realize that a shotgun or rifle is a vastly better weapon than any handgun out there. You may want to really think about trying to rent or borrow a few of each to feel it out. I spent thousands of dollars on handguns to one day realize that all pistols are short range emergency weapons. It was my Remington 870 that taught me that guns are not just weapons, but tools.
Hope I did not come off condescending with all the extra info. I'm not sure what you know at this point, so I'd just as soon be as helpful as possible.
