Depends on individual needs. At home base, where you can establish and maintain an ammo stockpile, I'm partial to .357 SIG. I like its street track record, its inherent accuracy, and its flat trajectory in the rural environment where I now live.
On the road teaching, definitely the .40. You can always find ammo for it (even during the last Ammo Drought, most places). .357 SIG ammo tends to be thin on the ground.
The beauty of these two calibers is that with another barrel -- and in some platforms, some more magazines -- you can swap between .40 S&W and .357 SIG in the same pistol.