I switched from .45acp to 357SIG and havent regretted it in the least.
The SIG does duplicate the 357MAG load that its was intended to (except for the blast and flash), and does so in guns that are more or less the size of a 2" J frame. The 357MAG load is based on a 4" gun. My P239 and S&W 60 are about the same size, but the 239 has a barrel thats an inch and a half longer, and its a hell of a lot easier to shoot well with. My 15 yard SIG groups are about the same size as my 7 yard S&W groups. Bump things up a little to my P229, and I get a gun that is just a little bigger than the P239, and carries basically double the ammo. Ammo runs a little hotter too with the extra half inch of barrel.
Ammo isnt hard to get, nor is it all that much different than anything else. Its actually cheaper than .45acp.