I've never shot the .357 Sig, but as I understand, it was designed to be equivalent to the .357 magnum which is also a snappy round.
From a reloaders standpoint, the Sig cartridge is a necked down 40 S&W case. I've never tried it, but I don't see why you couldn't take 40's and run then through a die and make your own.
The .45 ACP is much more available commercially and shooters have been casting and creating loads for years.
Now that I have read and looked into points brought up in the forum I think I am going to go with the G21SF. I was looking at the ammo cost on the .357SIG. Whaaaaat!
. 357 sig is one of the best rounds I have ever shot bit it's pricey. I had a Glock 31 with a 9mm LW conversion barrel. Sold it and upgraded to a sig p226 in 40 s&w and bought a 9mm barrel for it to keep practice coats down. I'm still looking for a 357 barrel for it at a decent price. Practice with the 9mm and carry the 357 sig.