Being a born and bred city boy, I think I'd take a Smith and Wesson model 317.
Having roamed the city, I think the light weight and concealability is of very high importance. But just as high, is can the gun also get you food. If I were homeless in my hometown of Washinton D.C., I could load the revolver with standard velocity shorts or CB caps for plinking off squirrels or pidgeons in Rock Creek park, or down along the Annocostia river bottom. Then loaded up with some good CCI ammo, it will do if pressed into SD duty at night while trying to sleep someplace. Any danger from two legged preditiors will be from other homeless trying to rip off my own stuff.
I feel the alloy revolver will stand up to the elements better with little maintance. It can be wrapped in some plastic bags and burried or stashed if need be. Most times durring daylight I wouldn't want it on me, I'll make do with a stick/cane. No laws to worry about then.
But I don't think I'd really bother with a gun if I were homeless. Too much of a hastle. What I really would like is probably a nice Webley air pistol that I can shoot a pidgeon with and not alert the whole nieghborhood. With a little salt, pepper, paprika, roasted over a fire, it tastes as good as a Cornish game hen.
Yeah, I'll take a good air pistol. Final answer.