never use a single if you can get a better choice.
1 most single shot 20 guages are hammer guns. in a moment of haste or terror, a hammer shottie is not really a safe choice.
Some will say otherwise but i have seen two near misses with hammer guns and newbie shooters.
2 Do you hunt? if you do, or maybe want to. spending a few bucks more on a remington 870 express will be money saved if you end up using the gun for more than one purpose.
3 get a pump gun. hundreds of thousands of cops use them and there are reasons. They are more reliable, easy to learn how to use, and very very effective. More effective than 38 revolvers, more effective than a rifle in the house. A sks is a wonderful rifle, but really not a good house gun. I sleep with a 12 gauge in my room.