I own a 12 gauge USH (SB2-980) and in my opinion does not kick that bad. Now after you shoot like 15 shots that is really when you start to feel the pain in your shoulder but compared to a the 870 the kick is considerably less. I had a Mossberg 535 shotgun which is some what similar to an 870 and all I know is when I shot my Mossberg 535 it kicked like a mule compared to an H&R.
However, I have read articles about through the advancement in technology that 20 gauge slug guns are actually more powerful than a 12 gauge at 200 yards; Now personally, I would go with a 12 gauge USH or a 20. Either way, you are going to be happy with how accurate that gun is but I would defiantly get a good scope for it because the kick on quality slug guns like the USH can sometimes break the scope because the kick on the gun.
Bottom line is, no matter which gun you get being a 12 or 20 gauge you are going to be amazed at how accurate it is. Last year, I killed a doe at 200 yards and double lunged it so it did not even go 15 yards. All I know is, once you start shooting and see what it does on paper at 100 yards you will feel more confident in your slug gun then you ever have felt before. You can't go wrong with a 20 or 12 gauge.