12 is much more popular and people buy, sell, trade-em all the time. 20-gauges I think people tend to buy when they really know they want one, keep 'em when they do, and grab nice ones pretty quickly when they come up used. My $0.02.
Right now I only have one shotgun, which is a basic 12 gauge pump; an 870 Express. I am looking into getting another 12, but an o/u for clays. If I ever get a third gun, it well may be a 20. Meanwhile, I can reload 12 gauge shells to mimic a 20 gauge load.
