Maybe a price range would help us out a little?
Lots of good and "cheap" shotguns out there...but if you want something to complete and match the rest of your collection it'll narrow it down some.

The 870 is indeed a good starter. But if you are worried about recoil the semis are worth your attention. The Mossberg 500 is a great gun. I'll never snear at it. It does what its intended to do excellently. I myself don't care for them though. They are usually only slightly less than an 870 but don't look near as good and they're balanced completely different. (Having said that the newer 870s and the older 870s shouldn't even be considered the same gun in my book) Dang it! I'm rambling again! lol

20 gauge vs 12 gauge is a moot arguement to me. Unless you want something for specifically for waterfowl and turkey. They'll both kill everything the other will kill and the amount of recoil is totally dependent on which two models you are comparing. 20 gauges produce less energy but they generally weigh less also...so you "can" end up with just as much recoil. I watched a 10 year old get put on his butt by a 20gauge pump with target loads[&:] Needless to say he didn't know what to expect!!!!

Give us a price range and maybe an action preference and then we can really start helping ya!!