[Rick]

I'll have to go out on a limb and state the obvious. You can pick up a box of .45 ACP pretty much anywhere that sells ammo. You'd probably have to hunt a bit to find .357 SIG.
Not to say that one is better than the other, it's the same issue I had when I got my first 7mm-08. If I'd gone the .308 or '06 route, I could have bought ammo at the gas station down the road, but since I had to be all trendy I ended up having to go to Grice Gun Shop in Clearfield to get it. ( I lived in Clearfield County at the time.) Sure it's more accurate, flatter shooting, better ballistic coefficiency, and can be used on comporable game, but, I got to be the guy with the "new" gun. That issue has worked itself out as the 7mm-08 became more popular, but I don't see the same trend for the .357 SIG. It's sorta like the 10mm was when it was introduced. All my buddies wanted to jump on that ship, it's so much more powerful than the 9mm. Then the .40 S&W came out, and pretty much destroyed the 10mm. A little slower, a bit less recoil, basically the same bullet. It caught on and the 10mm has fallen into obscurity.
But back to the question at hand, the .357 SIG is going to travel faster, pentrate more, and probably cause more collateral damage than the trusty old .45 ACP. So then you have to ask why not go with a revolver that is chambered for .357 mag? You could shoot .38spl out of it for a fraction of the cost to practice, and 6 rounds should be able to put the average nutjob in a 6x3x6 unless a whole lot of shootin' is required. I did read where a cop used his backup .357 SIG to kill the guy inside a vehicle because his .45 ACP hadn't done the job, if that's the kind of shooting you're talking about then go with it by all means.