Gun for street defense?

I recently received a concealed weapon license, as my Job as a PI is dangerous. Now, the only problem is choosing the weapon. The weapons the guy at the store recommended were the .28 caliber the 6.5mm, the .38 the 9mm, the .44 magnum the .45 acp, and a shotgun (sawed off). which (if any) of these is the best for street defense. Keep in mind there is a lot of gang activity where I live, so I might be fighting more than one dude.