Consideration
Though your poll specifies hardware, may I suggest a few considerations. The weapon best used by your wife is the weapon she can use the best. Assuming this has not already happened, go with her to a basic firearms course. Have her shoot several different guns. This way she can get the "feel" of different weps (revolver, semi, various calibers). In addition, she gains valuable coaching/instruction. You/she can then match her hand size, strength & dexterity to a wep. SHE has to be comfortable with it to use it. In general, for home defense consider a weapon greater than 9mm/.38 from a reputable manufacturer (Colt, Glock, S&W,...).
MOST important is training and practice-practice-practice. Home defense is an involved subject, it's not enough to have the wep. Go through a home defense course (NRA offers one). This way tactics (use of cover, flashlights, "clearing" a room, securing your children) and legal issues are brought up as well as marksmanship. On a regular basis spend time on the range. These are perishable skills. Her mind and skill have to get her through an ordeal and the aftermath, the hardware just has to go "bang" when needed.
Do you have a budget? You and your wife would be better prepared by buying a $300 (quality) .38, $100 in ammo and 2 courses than a $600-700 wiz-bang 9mm.
Another thought: Have you considered a shotgun? A Remington 870 in 20 gauge with a 18" barrel maybe fairly manageable for her.
-stick