I think it's all dependant on the size of the agency. My PD (6 full time inc. Chief and 10 part time) is pretty laid back about sidearms. Duty weapons must be .38 or better caliber, no magnums, no SA only, must be approved by the firearms instructors (me, since I'm the only gun guy). Off duty is the same except includes .380. About half the guys bought new .40 Glocks a few years ago, quite a few Glock 9mm, myself and another Officer (interestingly, the only female that works regularly) carry .45 Glocks (G21), a Sig 226 9mm, a Beretta 9mm, the Chief carries a S&W 642 (I think, the aluminum framed Centennial snubbie) and one single, solitary S&W M66 (that's loaded with .38+P ammo since no magnums are allowed). Issue, if you want one, is a Beretta 9mm or you can buy your own subject to the above guidelines. I'm working on the Chief to allow .357 for duty and off duty use, we'll see how that goes. The head instructor is lobbying the Chief to go to all Dept issue .40 Glocks if our budget problems ever ease up, will have a choice of G22 or G23. For backup/off duty, we have a few carrying .38 snubbies (I carry an SP101, others are various S&W), one carrying a G27 and some qualifying with their duty weapons for off duty carry. If the Chief approves .357, I'll be strongly considering carrying my 4" SS GP100 and the Officer with the S&W M66 will start carrying .357 in it. Depending on the duty round approved, I might even carry it in my SP.
The Ohio Highway Patrol issues .40 Sig 226 DAO and, from what I understand, doesn't allow ANY OTHER duty weapon. I understand they have some lattitude for off duty/backup but not much.
Another Dept nearby that's somewhat bigger than ours issues 9mm Glocks, either 17 or 19, your choice. They also have a much bigger budget than ours, this might factor into it, they used to have a policy similar to ours.
Myself, from an instructor's viewpoint, I can see the logic behind issued duty weapons. One caliber of ammunition to stock, one type of magazine to stock, one type of spare parts to stock, one type of holster to stock, one armorer's school to attend. However, looking at it from a gun nut point of view, I can't agree with the "One Size Fits All" philosophy. I know I have the utmost confidence in my G21 and would not feel that way with a .40 Glock or a 9mm Beretta. Others feel the same way with their personally owned weapons. Also, there are fit issues with issued weapons. The way my Dept works now, as long as the weapon fits the guidelines and is safe and from a reputable mfr, it will get approved, meaning that small females could carry, for example, a Kahr if it fit them well but a Glock or something else did not. If a Dept has fast and hard rules about sidearms, you're stuck with it, I'm afraid.
Bub