I usually carry an HK-USPc in 357 sig. The round is a bit of a boutique cartridge but the ballistics are there and it shoots very well. Being boutique, it's hard to find a wide range of ammo for it in local stores so that might be something you want to consider. I particularly like that it's head spaced on the shoulder as opposed to the case mouth or rim. If you reload this makes for a lot better control and less worry about case trimming. That being said, there isn't much neck to hold the bullet either which is a bit of a downside and was a problem with certain brands of ammo; especially Federal American Eagle, early on. I haven't seen any issues lately though and I think the manufacturers might have added a bit more neck tension.
Anyway, it pretty much duplicates 357 mag 125gr ballistics in a semi auto pistol vs having a revolver or desert eagle with the longer 357 mag cartridge. So, as a self defense round it's more than capable.
I like mine a lot and don't think I'd ever get rid of it.