I know my 2215 with a 2217 impeller has less flow than a 2217 but I wouldn't mind a tad more flow. I do have an inline diffuser (which didn't seem to affect flow much) and a UV sterilizer as well. I think most people would be happy with a 2215 on a 20 long but for me, I still would want more.
I think it's always better to go bigger as you can use the filter on a larger tank and their really isn't a downside. My 2234 was fine for a 20 long, but throttled back 50% still works on my 6 gallon, though I am not using it because it is just physically to large to fit into my makeshift stand.