I shoot the .40 and I like it. I have the 229 and an interchangeable barrel so I shoot both .40 and 9mm.
For me there is not much difference. Can't speak for the 357 sig - I've never shot it
With the P229 and P226 if you buy the 40S&W model you can purchase a 9mm barrel and magazines and fire both. Also the .40 and 357sig only requires a barrel change - you can use the same magazines.
Stopping power of the .40? - I'm surprised that hasn't gotten a reply. lol