Over the years, I've had both, and I still shoot my 20 ga Ithaca that I got 45 years ago, the best. I can really hit with that gun.
My wife has a 20 ga single shot that she's also really good with. My brother has a chrome barreled black stock single shot that I give him endless grief about, except that he can out shoot me any day of the week.
I like everything about a 20 ga., it's lighter, smaller, and easier to shoot. Maybe a 12 can turn a bad shot into a hit, but I regularly out shoot guys with 12's. Back in the days when we had pheasants around here, I alway got more than my hunting partners.
Sooo, I'd get her a 20