I'm a shotgun instructor as well as a hardcore waterfowler.
a 3 inch duck load will have more recoil than a 12 gauge 2 3/4 in duck load due to a couple of variables. you will also run into a finding decent 20ga loads during season in most cases. a 2 3/4 inch 12 gauge load will normally pattern better as well.
get her a 12 and shoot 2 3/4 loads.
in my opinion an 11/87 is the softest recoiling and best naturally pointing shotgun out there. find a 26 inch 11/87 with a wood stock and take it to a competent Smith for fitting with hunting clothes on. it will make all the difference in the world.
we have dozens of SA-80's at our BSA Range. not bad guns, made in Turkey.. I do what I can to avoid Turkish guns. a 1100 or 1187 is a better overall gun. parts are everywhere and you can repair yourself in a lot of cases.
if you are dead set on a 20 and she is smaller in stature, you cannot go wrong with a 1187 youth. my son had excellent results with FASTsteel and Federal 20ga 3 inch #4 with a Briley extended IC or Skeet choke.
don't waste your time with #2 shot, we couldn't get decent patterns.
I see you are in DC, if you are shooting divers you definitely will want a 12 to deliver 3 or 2 shot. we have not had good results with 20 gauges on Divers with #4 steel.
here is a 1100
1100
11-87
both are excellent guns.
I would try to find a 24 inch barrel model if I could. my son had a 21 and a 24 inch and he, along with his friends almost always shot the 24 better.
Good Luck-
keep me posted on what you pick and how she does. I am a scattergun junky and always looking for real world info.