I have the Sig P229 with both the .40 S&W and .357 Sig barrels. I like them both, but I mostly use the .40 S&W. The ammo is a lot cheaper for the .40 and easier to reload. With the P229, the .357 Sig round actually feels like it recoils less. One thing I've noticed is that the 2 rounds have different points of impact. The .357 Sig hits the target significantly lower than the .40; probably due to being a faster round and exiting the barrel quicker.
Although I only carry with the .40 S&W (because that's what I train with), I feel confident in the stopping ability of either round.
I do not own a Sig (yet), but I do own 2 Glocks in .357 Sig caliber.
The stories you may have heard about recoil are greatly exaggerated. I have a full size Glock 31 and a sub-compact Glcok model 33, both chambered for .357 Sig. They are FUN to shoot.
Several weeks ago I was in a local gun shop where a fellow was purchasing a Glock 33 and the employee behind the counter was actually laughing at the customer and telling him how much it was going to "kick".
I could not believe the employee was saying that to a customer.
Ok... Back to your question... The .357 Sig is very powerful and very fun to shoot. On the other hand the ammo is very expensive. Keep that in mind when you make your decision.
I have a 9mm barrel for my Glock 31, along with a .22 conversion kit. I rarely shoot .357 Sig any more, it is getting too expensive for me.
I have a P239 in .40 S&W and I love it! I've never shot the .357 sig round, but I've considered buying the .357 barrel, it only costs about $100. That would be my suggestion, buy it in the .40 S&W, and then buy the 357 barrel. Then you got the best of both worlds.
One more reason to buy the gun with the .40 barrel, then the .357 separatly is that the .357 Sig round will fit in the .40 S&W Magazine, but the .40 S&W round will not work in the .357 Sig Magazine.