Go to the range and test. Make a selection/Choose a weapon. Then, whatever you have chosen to buy for protection, buy a second one in 22LR. This way you both can practice your butts off and be familiar with all the operations of “the serious one.” Of course, once you know what you’re doing with the 22LR, practice a little with the larger caliber so the recoil won’t catch you off guard in an urgent moment. The money you’ll save in practice ammo will end offset the cost of the second handgun, over time.
If you can’t match the handgun to both calibers, I’d consider a revolver. There’s less to think about and you can be faster than guns that present a safety switch step. Everyone will scream about a revolver having less shots. Understand, most attacks happen within 10′. It’s not a Hollywood shootout. The less you have to think about, the better, which why some others have wisely advised to shoot, shoot, shoot. However, you should also practice, practice, practice real time situations and scenarios.
Another fantastic option is a defense 12ga shotgun. Way more effective than any handgun, but not as concealable as something that fits in a drawer, cabinet, or box. Still, every cop will tell you, this is the best home defense there is.
Lastly, I disagree on the longer barrel recommendation. Shorter barrels push backwards less. They tend to lift more than push, which makes for a more manageable recoil. Think in terms of a head-on collision vs an angled collision. More of the energy is deflected because of the lift.
Once upon a time I had two Taurus Judges: one with a 6.5″ barrel, one with a 3″ barrel. I sold the 6.5″ because it jarred my wrist instead of rolling in the grip.
Again, go to a range and try everything. You can’t know until YOU know. Fit, feel, action, and it has to fit two people, if you’re going to share. Realistically, there’s wisdom in finding what fits you best and what fits her best. Yeah, it costs, but doing things “better” often does.