I agree with drsfmd on the O/U comments. I would not get one of the cheaper O/Us as everything I have read is that they don't last. The Ruger Red Label may be an exception, I know the old ones were good, the jury is still out on the new ones but everything I've read has been positive. I'd like to see a long track record of 100k+ round guns before I give them my vote, though.
I disagree about 20ga being a handicap. From a purely technical standpoint, there will be less lead flying at the bird, and therefore less probability of a hit. But if you do your part, you will hit the bird regardless. In other words, if you are dead on, it will not matter whether there was 7/8oz of shot or 1 1/8oz of shot. Of course we aren't always dead on, so you have a slight edge with the 12ga, but I would not let this influence your decision at all.
I also think a 28" or better barrel is acceptable. I have shot trap with guns ranging from 18.5" to 30", and of course I prefer the 30", but did not find that anything 26" or above was a significant problem. I have a friend who shoots with the standard 26" 870, and keeps up with me (although that may be more of a comment on me than him). My Citori wears a 30" barrel and I couldn't be happier.
Might I recommend looking into a semi-auto shotgun. You can get a decent one without spending a ton of money, which can't be said about O/Us. We got my girlfriend a Weatherby 20ga (model SA-08) at the beginning of this season and it has been outstanding. I believe it was around $400. They will also have less recoil.