In my area my dept. and most of the local town PDs carry the .40 cal Glock so I voted for the .40. However, the city pd and the state police carry 9mm Glocks so there may actually be numerically more 9mms.
To be perfectly honest, I don't know what my local officers are carrying. When I see uniformed officers, some are carrying Glocks in 9mm, some in .40, and some are carryin S&Ws in 9mm or .40. Occassionally, I see an officer carrying a Sig in .357 Sig or .45 ACP.
I'm going to opt for .357 Sig though, since I know it's the current issue round for Texas DPS.
Locals carry .357 SIG, .40 S&W, and .45 ACP
State carries .357 SIG
Federal agencies mostly carry .40 S&W. Among those given a choice, .40 seems to be most popular, with .45 next and 9mm a distant third.
Based on the San Antonio city council's procurement authorizations, looks like the SAPD uses .40 S&W 155gr CCI/Speer Gold Dot Hollowpoint. Their issue pistol (according to the SAPD Armory website) is the Glock 22, with its little brother (23?) authorized for carry as well.
Just an update...this evening while at dinner I noticed three City of Plano(my locale) officers in the restaurant. ALL three were carrying cocked'n'locked 1911s! I was very surprised when I noted this as the general belief seems to be that cocked'n'locked 1911s are no longer found in duty holsters anywhere. I would presume that all of these 1911s are .45 ACP.
Later at my local mall I noticed an officer with a 9mm Glock 17 in his duty holster. Again, a City of Plano officer. So, I can only conclude that officers purchase their weapons(as the 1911s were customized) and can carry 9mm, .40, or .45.
The big National Guard base up the road from me has its own police force, and one of the officers there carries a well-used Kimber .45. I first met him in a gun shop (SURPRISE), and he cleared his weapon to let me check it out. Like I said, it had been used a good bit, but I noticed that some little personalized bits were added by someone who knows a heck of a lot about handguns. He was loaded with 230-grain Hydra Shoks.
Seems like Houston is a bit like Random Man's observation about Plano: Most LE seem to have their own guns instead of a dept. issue. One sees Glock, S&W, Sig, etc. once in a while a 1911, once in a while a revolver still on an older officer's belt.
McAllen looks like there are many .40 Beretta pistols. In Edinburg the Glock seems nearly ubiquitous.
Texas DPS, as others have mentioned, uses the .357 Sig.
It was my understanding that back when semi-autos were first allowed for the PD in Seattle, the older system had .38 or .357 armed LEO (higher qualification scores and one could carry the magnum). Cops with high qualifications could carry any semi-auto that they qualified with, even saw a .45 S&W back then. Now I think it is .40 Glocks. My understanding is that Tacoma has gone to Glock .40 pistols or .45 1911 Kimbers.
State patrol, city police departments, and Div of wildlife officers in Colorado Denver/Boulder and suburbs carry either .357 mag, 9mm, .40 S&W, or .45 ACP and a variety of handguns too. Most carrying semi-autos are carrying Glocks it seems. I've seen anything from Colt and SmithWesson revolvers to Glocks, SIGs, S&W Autos, Berettas, even seen some cocked and locked 1911's and Browning Hi Powers.
Boulder PD issued the Federal Tactical Hydra-Shock. I believe Denver issues Speer Gold Dot
The city here just went from .38 Special to .45 ACP using a SIG platform. As they issued a .357 S&W but restricted the round to the .38 and said it was good enough, makes one wonder why the change to the .45.
Local sheriff issues 9mm/.40 Sigs. Several cities do likewise. One city near here issues the Glock 22 (.40). Most state agencies (Fish and Game, DOJ, DMV, Heath Services Medi-Cal Fraud Squad, ABC) issue .40 Glocks...many recently converted over from 9mm. CHP issues the S&W 4006 (.40) and has done so for many years. In past years, the state Bureau of Narcotics Enforcement (BNE-Dept of Justice) had a pretty open policy regarding handguns, and when I went on a raid with them on one occasion, seemed like just about everybody was carrying C&L 1911s...but that's changed in the interest, I guess, of uniformity to Glock .40s. Incidentally, the California Attorney General is very anti handgun and probably had something to do with converting over to the Glocks so no one could see those nasty cocked hammers, but a local range here banned DOJ agents from using its range to qualify as a protest against the AG....individual officers can come in and shoot, of course, but the range will not allow formal DOJ quals or training there....
FYI from a recent Seattle police shooting...2x .40 S&W Glocks. I had heard that focus on the hands and immediate threat resulted in cops often hitting assailants in the weapon and/or hands, but the photo is a new one for me...