My opinion? The 38 special. Depending on the gun, of course. The 357 is arguably the BEST self defense cartridge available, if you use the 125gr HP loading.... BUT... the muzzle blast and flash is borderline horrific. Recoil in most handguns is enough to make a second shot pretty slow (if you actually aim it

).
The 38 special is powerful enough to do the job, but mild enough that most women or "non-gun people" can shoot it with very little practice...
Ammo for the 38 is a little cheaper, as well. If you bought a 357, of course you could shoot 38 specials out of it.... exclusively, if you wished.... and you would always have the option of using the 357 if you so desired.
You can usually find 38 special revolvers a little cheaper than the comparable revolver in 357 magnum, but if that is not an issue, then I would recommend a 4" stainless revolver in 357 magnum, with the intention of shooting 38 specials in it most of the time.