I have a P239 in 357SIG. Its a good pistol, but so close to a P229 size wise, I'd choose the P229 and the extra 5 rounds in the mag over it.
The factory grips on the P239 are a little skinny for my taste and I replaced them with a set of Houge checkered coco bolo's. They give the right amount of "swell" in the palm. To bad SIG doesnt offer a set of factory grips with the swell too, I'd be all over them.
357SIG is (or was) readily available online, and is always on the shelf in both Walmarts near me. It and 22-250 were actually the only two boxes still on the shelves the last time I was in. Cost wise, its the same as .40 when you buy it in bulk.
I have spare .40 barrels for most of my 357SIG guns. I got them as a back up, in the rare chance 357SIG wasnt available, and really only use them occasionally. I prefer the 357SIG over the .40 and thats what I keep in the gun.