I would suggest that you get a Glock.
A backup gun in an ankle holster is more likely to be exposed to dirt and the elements, so I would suggest a revolver. You can have a small, lightweight J frame .38, and put some more hand-filling grips on it to accomodate your grip. Larger grips will make the gun a tad harder to conceal, but if you choose smooth or checkered wood grips, so they don't catch your pant leg like rubber grips, it can be done. Depends on your pant style, really.
I don't like ankle holsters, but if I had to wear one, I'd go with a Galco Ankle Glove. It is the least uncomfortable ankle holster I've ever tried, though I can't stand any gun on my ankle, even an airweight. It just bugs me. YMMV, of course!