If you buy in bulk, the price difference between 357SIG and .40 is minimal. Last time I bought it, it was exactly the same, but that was back before the gold rush.
I have SIG's with spare .40 barrels, but they rarely get used. They are a true drop in barrel though, and all thats needed. Magazines are not always interchangeable though. The SIG P239 has caliber specific mags, and while the 357SIG will work with the .40 mags, the 357SIG mags have an offset in the mag at the case neck on the round, which will keep .40's from being loaded into the mag.
A question for you Glock 31 owners. I recently picked up a used 31 which was basically, as new. I notice now as I shoot it, the locking block is impacting the underside of the slide and peening it in two places. The marks were not there when I got the gun, nor was there much of a smiley on the barrel when I got it, which leads me to believe it wasnt shot much. I've got about 500 rounds or so through it now, and its obvious its starting to pound it a good bit. I've been told this is common with the 357SIG and .40s, but I've yet to see it in any of the .40's I've seen that I looked at (2). Anyone else have this going on?