Given the same 'throw weight' and velocity, recoil will be greater with the lighter gun. It's simple physics at work, every action has an equal and opposite reaction. Recoil is reaction.
Don't dismiss the 12 gauge too soon. Choices of ammunition are MUCH more limited for the 20 than for the 12. For that reason alone the 12 might be a better choice, as reduced recoil loads suitable for defensive use are almost impossible to find in 20 gauge. And a lighter gun in 20 ga. will kick worse with heavy loads that a heavier gun in 12 ga. Might sound like a contradiction, all I can say is find someone who has the guns you are looking at buying and try them yourself.
Proper gun fit and proper form are BIG issues in recoil management for shooters. Many shotgun stocks are too long as they come from the factory for smaller shooters. MAKE SURE the gun you get is a proper fit FOR YOUR WIFE. It is far easier to shoot a shotgun with too short a stock than one that is too long, just remember to put your shooting hand thumb over on top of the knuckle of your trigger finger when shooting. The first time recoil causes you to bop yourself in the nose with your thumb in the usual position you'll see why this is necessary, and the lesson won't likely have to be repeated. I'm 6' 3" and my wife is 5'4", and we both shoot the same 'house guns-' Remington 870 12 gauges with 18-20" barrels, full length field type forearms, and stocks with 12 1/2" length of pull (measured from trigger to butt).
Form is important also, someone who knows how needs to coach you both in assuming proper stance, proper gun mount etc. Get some help on this one, especially for your wife. Unless you taught her to drive also, that is. If you taught her to drive and you're still married, you might just be able to handle teaching her to shoot. 8^)
I'd suggest you look into a Remington 870, just because the gun is a bit heavier than the Mossberg. Weight is your friend when reducing recoil is the issue. Shortening a stock is no big deal for a gunsmith, neither is adding one of several brands of premium recoil pad that the gun should wear (KickEez, Limbsaver, Remington R3, etc). A properly fitted all steel 12 gauge with a premium recoil pad and good reduced recoil loads is likely your best bet. But experiment around some before you buy, rent or borrow different shotguns and get some trigger time with your wife to help make the decision.
The lightest kicking buckshot load I have found so far is Fiocchi reduced recoil 00, stock #12LE00BK. Federal and Hornady ('blue hull' TAP) make good loads also.
hth,
lpl/nc
