If you're not going to carry, or only going to carry it open at a range or hunting, anything from 4-6" works well in my opinion. I will disagree with the idea that anybody who tries a 6" and a 4" will gravitate to the longer tubes; I've fired plenty of 6"-7.5" .357s and both of the S&Ws in my safe are 4" tubes. The longer barrel is easier for most people to shoot accurately but with practice and proper technique a 4" gun is just fine out to 50-yards.
As for performance, I'm a huge 9mm fan (have been for as long as I've been shooting handguns) but the .357 can outmuscle the nine. My Glock 17 clocks 1270-fps with 127gr +P+ Winchester Rangers; experience suggests the G-26 should clock around 100-fps less with the same ammo. My 586 clocks 1450-fps with run of the mill Rem/UMC JSPs and I would be really shocked if a 3" gun was magically less than 1200. And at the same time I can comfortably toss 158gr .357s at 1200 fps with a 4" gun; nothing in a service-sized 9mm can do that. Though I will say the 9mms are easier on the shooter than the Magnums.
In the end though, pick the one you like and use up lots of ammo getting proficient. There aren't truly any wrong or bad choices (except maybe the ultra-light .357s with full-house MAGNUMS. You're on your own there, I'll take a pass...
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