The older I get, the more I come to like the lighter shotguns for which the 20 is chambered.
Some years ago I picked up a SxS Stevens 311 in 20 gauge and I am absolutely in love with it.
I truly believe that the 20 gauge has its place in a shotgun arsenal right alongside a 12 gauge.
Of course yes, there's some truth to the saying that there's nothing a 20 can do that a 12 can't do better...
Same could be said of a 10 gauge... Or an 8 gauge...
I've never been a big fan of over unders, that's just a personal thing on my part, but my suggestion would be to, if you can, handle both the 20 and a 12 and see which one mechanically works better for you. There can be a lot of different in how they handle.