Yes..the 870 is a great choice, I have two of them, one is my main hunting shotgun with beautiful walnut stocks, so I wont be cutting that one down. My second is an 18 inch 870 HD model. Box stock with nothing on it but a butt cuff with 4 rounds of Hornady TAP buckshot.
I also have a yard sale Savage pump gun that had the bbl cut down to 18 inches before I got it. I put a slip on recoil pad on it, and I keep it stashed on the first floor with buckshot therein.
As for a SAIGA 12, I think a lot of it, but I dont have the cash lying around to buy one because I need to spend my gun money on 45 auto ammo, 9mm and a lightweight 38 J frame model 642.
The saiga is well thought of on www.warriortalk.com by the folks who are trained by Gabe Suarez. I also saw a former member of the Spetznaz named Sonny Puzikas who really worked over a combat course with a saiga with a 10 round mag in it.
I may get the saiga this fall if I can get my other wants taken care of.
Do yourself a favor with the 642: get the Crimson Trace laser grip. You don't have to buy it that way, the grip can be ordered via Midway and a host of other places. The 642's sights, to be highly technical, SUCK. It is a concealment gun, meaning that the sights are very low to the gun, and the sight radius is very short. Plus, the only time you're likely to use it will be in conditions of low or no light - and if you ever need to use it, you'll realize that the CT is worth its weight in diamonds.