I would recommend nothing less than a .32 for defense. There are plenty of light, small, high quality guns in that caliber and the recoil is not too much for your wife to handle. If she's able to control it you should consider a .38. The more power you can pack the better off you are. Visit a gun range and rent a small pistol your wife can hold comfortably and let her shoot a box of ammo with it. It will run you about $35 or $40 but she'll get the feel of the caliber she likes and involving her in the decision will ensure she'll be happy with the gun.
I'm a wheel gun believer myself. There's a lot less complication involved with them and reliability is important. I've had too many jams with semi autos and a new shooter will have less trouble with them. Just my 2 cents.
I don't know who could argue your point about having a revolver for self-defense, especially for the wife who, if she ever needs to use it, needs to just pick it up and shoot. Simplicity is what is needed at that time. May not be as sexy, but who worries about sex during a home intrusion? (Other than a potential raper)