At my small Dept (6 full-time, including Chief, and about 10 part-time), as long as you abide by the Dept firearms policy, anything goes. Policy is as follows:
Duty gun- anything .38/9mm and larger, no Magnums, no SAO, reputable manufacturer and subject to approval by one of the firearms instructors.
Backup/Off Duty- Same as Duty Gun except .380 is included.
If you want a Dept provided gun, you get a Beretta 92FS 9mm.
As one of the firearms instructors, I can see the wisdom in either approach. Officers tend to shoot better, maintain better and have more confidence in guns they choose and personally own. I have benefited by this and carry a personally owned Glock 21 as my duty gun and my personally owned SP101 as my backup/off duty gun. Glocks in .40 are by far the most popular choice, but there is one Sig P226 9mm, one Dept issue M92FS, several Glock 9mm, another G21 and one S&W M66 revolver. Hardly anyone carries off duty, with my SP101, a S&W M60, a new Ruger LCR and a couple G26s being the sum total of off-duty guns anyone qualifies with.
However, as an instructor and the person responsible for ordering ammo and maintaining Dept guns, I can see the wisdom of a uniform, one-gun-fits-all choice. Currently, I have to stock ammo for .38+P, 9mm, .40, .45 and 12ga slugs and 00 Buck. If the Dept standardized on, say, 9mm Glocks, with a choice of G17, G19 or G26 as the Officer chose, then I would only have to stock 9mm and 12ga ammo. In addition, all maintenaince needs would be covered by a Glock armorer's course and sending the Rem 870s in for service. As it stands now, unless the gun is a Glock, it has to go back to the Mfr for service and the Officer has to qualify with and carry a M92FS till it gets back.
I suppose a happy medium would be choosing a common caliber and having a choice of 3 or 4 different platforms for duty use, like the NYPD does. However, the individualist likes the way my Dept is currently set up and I can, basically, carry whatever I want. I'd like it better if the Chief would change policy to include SAO and actual .357 ammo (I'd LOVE to be able to carry a 1911 and .357 125gr JHP in my SP), but you take what you can get and be happy.
Bub