The .357sig is a popular caliber....That is what the original question is correct?
It's really regional though. People like to carry what the police in there area carry. since most Police Depts. carry .40 that is what is most popular with the public. Texas for instance DPS carries .357sig in 226's. Most everybody else carries Glock 22. Guess what the two most popular calibers are in Tx. .357sig, and .40S&W (.22lr, and 30-30 win. actually) For carry though its .40, and .357sig.
There is a trend in L.E. SWAT starting where heavy sub-sonic 9mm is getting more, and more popular again I think its a combination of suppressers sweeping the nation, better ammo, training costs, Etc. The truth of the matter is that at any "team event" your handgun is secondary and when ALL handgun calibers suck it really is "splitting hairs". It would be interesting to see how many patrol officers follow suit, since the same influence is there that I spoke about with the public. Officers tend to want to carry what SWAT carries.