dlcra:
To echo (and sometimes disagree) with WR Moore: A semi-auto shotgun is a LOT easier to handle with all but the lightest loads. I hate my Mossberg 500 on that basis - I'm relatively large, but still don't like it. OTOH, there aren't any parts it there

.... Not much to break.... If it's a "his & hers", she's going to have to really get into it.
("Tactical" or low-recoil loads are also a very good idea, either way.)
Birdshot v.s. slugs or buckshot.... Just IMHO, but birdshot may be safer indoors in most modern houses. I'm afraid that I'll punch through a wall or two with buckshot or slugs.... It also fits Joe Biden's suggestion - might as well just make a lot of noise

.... If you do hit somebody with birdshot up close, you should still STOP all but the most determined attacker. The problem is that STOP is iffy, and shielding (standing behind a door, perhaps) may be a little help for the BG. A 20 guage may also be a little more comfortable to shoot for both of you. I had one years ago, and managed to hurt myself with it (somehow the safety lever tore up my thumb), but it was otherwise a lot more fun to shoot.
The M&P will be fine for about any use except target work. It's a little harder to conceal than the Shield, but you can dress around the gun a little bit. They're just not that big. The Shield is about the same size, but the grip frame has a thinner profile. This does conceal better, but in .40S&W may be a little less comfortable than you or your wife prefer. Either way, the 40C is fine for just about everything else in terms of self defense.
Just IMHO, most anybody should be able to handle the 40C, although there may be a learning curve, especially if you or your wife are quite small. The 9C would almost be as useful, and is a tad more comfortable to shoot. I still like the .40

.... I don't think an average sized person will have any issues with the .40C that a little practice won't fix.
IAC, don't be put off by some of the responses here - each of us was a beginner once, and it's not the same as popping a DVD into a player! Do take the trouble to take some courses of some kind, and get a PA License if you haven't already got one (and get your wife to do it, too!). (I can't get my wife to do that....

)
Regards,