What is she using it for? Also you'll want to see how well a given gun fits her, as a poor-fitting gun will hurt more in general. The 20 is more versatile, the .410 can still do a lot, the 28 is great for birds, clay games, etc. Are you thinking single shot, double barrel, pump action..? I love my .410s, but if she wants to go deer hunting I'd go for the 20. If she isn't really into guns and you're picking it out for her you'll still want to bring her shopping with you, preferably to a range where she can try a few guns and you can find out her preferences.