whatever you choose...
1. She absolutely has to be able to get it going. Can she jack the slide on that Ruger, or is she carrying with one up the spout?
2. She's gotta be able to hit with it. Doesn't matter what gun or caliber it is, if she can't hit with it, forget it.
3. Awareness is a very large part of the equation, perhaps the most important part. Lots of women carry in their purse, but if she doesn't have it out when she needs it, it isn't gonna matter what she has---mace, taser, gun. A woman in these parts was kidnapped at twelve noon in a parking lot in a good end of town, simply because she had no idea the goon had targeted her until he had her in her car.