I had a .357 conversion barrel for my Glock 23 (.40).
It shot great, very snappy (Might be abit too much for some), was VERY fast, shot at a bullet-proof vest, it was the only round that fully penetrated, the .40 and .45 only tore some of the Kevlar, but didn't fully penetrate. The vest was strapped onto a tree stump (To semi-simulate a real body), and used XTP reloads on all 3x pistols.
Anyways, I ended up selling mine, I just prefer my .40 Also to reload the .357 Sig, can be quite a pain. You gotta lube, since they don't have carbide dies for necked brass, and just a real pain to make them cycle near-perfect, whereas my .40 barrel, will eat up everything, hence it being a straight-neck round.
Plus IF you're not a reloader or get the ammo free, you'll pay out the butt. I highly suggest you reload if you get a .357 Sig, then it's the same cost as .40, 9mm, etc basically. Just abit harder to find brass, esp laying at the range.
Overall, I'd say get a 40, it does me great. Has the perfect power ratio, and easy to find.