Best advice I can give would be for both of you to get your hands on any shotguns you're considering for purchase, and at least handle them first if not actually shoot them if possible. Could be family or friends can help out in that regard, or various ranges/gun clubs might have shotguns for rent. But look around for possibilities in that regard, and if at all possible try before you buy.
I don't know how physically large (or small) either of you are, your overall state of health/fitness, or your overall experience level with firearms. Without knowing more about the both of you it is hard to offer worthwhile advice on what is essentially a very personal question. So I will stick to some generalities here.
"Fit" of the shotgun to the shooter is one of if not THE critical element in successful shotgun shooting. Overall weight of the gun, balance and handling characteristics are important too, but they do not surpass gun fit as a critical element in the relationship of shooter to shotgun.
As a rule, most shotgun stocks are too long for many shooters. It is easier to shoot a defensive shotgun that has a too-short stock than one with a too-long stock. I generally advise setting up a defensive shotgun to fit the smallest person who will be using it, and encouraging everyone else to learn to use the shotgun 'as is.' I don't like any of the recoil reducing stocks I've tried so far, for a variety of reasons, and neither does my wife. As a result all the defensive shotguns here are conventionally stocked, but shortened to fit her - she's 5'4", and the house guns here have stocks with a 12.5" LOP. I'm 6'3" and wear a 37" sleeve and can shoot the short shotguns perfectly well - in fact, I have come to prefer them.
I wouldn't worry too much over the 12 vs. 20 gauge argument. Just get what both of you can handle easily, either will do the job with the appropriate ammunition. The 20 ga. guns tend to be lighter and easier handling, but with really heavy loads recoil can be more objectionable than in a heavier 12 ga. gun. But recoil can be a subjective thing, too. Good gun fit, good form, a solid gun mount and good technique will help a lot with recoil. Still, there's no reason not to take full advantage of the excellent recoil pads currently available, too.
I would stay away from any kind of pistol grip stock on a Mossberg. I find that it gets in the way of the easy use of the safety and action release in almost every case.
Be sure you look at and handle the various manufacturers' 'youth' model 20 gauge shotguns available these days. I have a soft spot for 870s, and there are different Youth and Junior models of this shotgun available. I think the Junior models are only available at Gander Mountain, I haven't seen them anywhere else but I could be wrong. See
http://www.remington.com/products/f...shotguns/model-870-express-jr-specialrun.aspx for details. The gun is 20 ga. with an 18.5" VR barrel, RemChokes and a 12" LOP stock. It and Mossberg's Bantam are the smallest production pump shotguns I know of.
hth,
lpl
