If I were to become a cop and I was to furnish my own firearm out of my own funds AND I could carry whatever I wanted I would go with one of two different philosophies (and I'd really probably alternate between the two if I would be allowed):
1) Unlike "civilian" CCW and home defense usage where you only draw when you will likely need to actually fire, a LEO actually uses a gun sometimes for its deterrent factor and draws much more liberally. So, deterrent factor is important. With that in mind I still think the revolver should be considered a viable law enforcement firearm. There aren't very many deterrents better than 6 (well, the BG would only see 4 of them, but he'd probably be visualizing the rest) Speer Gold Dots actually staring back at him. I'd be OK with a .38spl or .357mag for that, but to maximize deterrent value (LEOs do carry openly so conceability doesn't matter) I'd probably go with a .41mag, .44spl or .45LC. To back it up I'd probably use a Kahr P9 or PM9 for those times when you just need the faster mag change.
2) An auto is pretty much the standard LEO sidearm today due to faster reloads and higher capacity, a LEO is more likely to need the higher capacity than non-LEOs are. A LEO gun needs to be durable and built like a tank (it may see abuse, it will be exposed to the elements, and it still needs to work). To cut the weight a tad (you'll be wearing it all day) I think I'd prefer a slightly lighter gun. I think I'd prefer a .45ACP with at least 8 rounds or a .40S&W with at least 11 in the mag. Also, if I'm buying it myself cost must be considered (a LEO, especially a new one, isn't making that much money). I'd consider (pretty much in order) the Ruger KP345, the Taurus 24/7 (either 15+1 rounds of .40 or 12+1 rounds of .45 in a lightweight and relatively compact package is quite nice), a used alloy S&W 3rd gen auto in .40 or .45 (probably the DAO and definately without the rails), the new Taurus PT1911 (an affordable aluminum alloy 1911 will be nice, assuming their 1911 proves reliable), my Taurus PT140 Milennium Pro (I really love and trust this gun, and I already own it), a used Glock in .45ACP, or a more expensive SIG or HK. If the department was paying for it I'd consider the same thing but the SIG would probably move up the list and the 24/7 down a little and I'd drop my Taurus PT140 from consideration (it is a smallish gun, and if they're buying why go with something I already have). If I went with the auto it would be backed up by either my Taurus 85CH, or should I get used to the gun my S&W 442.