Once again, thanks for all the good info. Figured I'd write a brief follow-up.
Based on what I read on this forum, elsewhere online, and input from friends, I decided to consider only 12ga. It seems to offer the most versatility, and you can use lighter loads if recoil is a problem. I didn't hear anything actually bad about either the Mossberg 500 or the Remington 870, so I figured it would be a matter of personal preference at that point. So yesterday we went to Cabela's to check them out. That place was hoppin' and their inventory was a bit low, so they didn't have any plain Jane varieties of either. Instead, we got to handle the Mossberg 535 ATS Field/Deer Combo and the Remington 870 Express Super Magnum with laminate stock. Both felt good. There's nothing quite as satisfying as the sound and feel of pumping a shotgun. My fiancé made an indecent joke about that, and maybe there's a grain a truth to it. Don't know, but thought you might get a kick out of it, too.
At any rate, we both preferred the Mossberg for a few reasons. As many have pointed out, the placement of the release and the safety are in much more ergonomic locations. You don't even need to change your hand position to use those controls. Also, the solid hardwood stock had better fit, finish, and feel than the Remington's laminate. Whether or not it will matter much in use, my fiancé prefer's the two-bead sight on the Mossberg compared to the 1-bead on the Remington. And I know this goes against my initial set of 'requirements', but one of the deal breakers was the deer/field combo. The Mossberg 535 accepts up to 3 1/2" shells and it comes with two barrels: a smooth bore for shot and a rifled bore for slugs. And with all the deals and coupons I lined up, it came in at under $350 new. At a price like that, the 'versatility' requirement wins out.
Once I get it out to the range a few times I'll close the loop to let you know how it works out.