Some good advice here, some ballistic info not so factual. For some good info on shooting statistics regarding one-stop shots visit http://www.handloads.com/misc/stoppingpower.asp?Caliber=0

Using hollowpoints, I wouldn't worry about over-penetration in any caliber.

For some discussion with a bunch of pistoleros go to http://www.stoppingpower.net/

Statistics, and a lot of them, say the old .357 Mag is the best fight stopper around. Still, If you feel comfortable with an auto, I'd go with one. I'd look for a .40 cal Glock used. A .45 would be OK as well. Lots of police trade-ins around at decent prices. I'd also get night sights. The Sigs are more expensive but no more reliable and the Glock has all the accuracy you need for self-defense.

An auto not only carries more rounds, it's a whole lot easier to reload in a fight if you ever have to. Another thing to consider is that if a gunfight takes place around an auto, the .38's and 9's won't penetrate doors or glass like the heavier slugs of a .40 or .45.

I have carried for a few decades. I'm partial to bigger bores, and own about every caliber around, except the super mags. I find myself leaving my .45's in the safe and carrying a .40 cal Model 27 Glock compact most of the time with a .32 Kel Tec in a wallet holster-- just in case.

A 30/30 or cheap SKS behind the seat is a good poor man's assault weapon as well. And you simply can't beat a 12 ga with buckshot for a close range weapon.