For Police use I find it is practical to carry more than one light. That way, depending on circumstances you will be prepared.
dougmccoy was right, for police use it's worth spending the money. Don't want your light to fail at the worst moment.
For large areas, traffic stops, night time when I need lots of light I alternate between two lights. My SL-20X-LED which also has the LED function for vehicle searches or lower lighting needs. A very versatile flashlight all round, a little big for carrying on your person so it usually stays in the car or bag until needed. Rechargeable so no need to carry extra batteries all the time. Depending on area it's in the $150 Canadian range. I also use my MagCharger (slightly modified) for this purpose, although with my modifications it's getting a little too bright for interior vehicle searches, but great for large areas outdoor at night.
For on the duty belt/person, two lights are practical. I have a Streamlight Strion, rechargeable, perfect size for the belt, always with me. Has good throw and spill, power if needed, and rechargeable so no need to worry about batteries. Adjustable beam for throw or flood depending on the need. Tactical tailcap switch. Momentary on (tailcap switch) or constant on (Twisty tailcap). depending on area it's in the $100 Canadian range.
On the belt as a backup, Pelican M6-LED. Good runtime if I need light for a long time. I figure if I burn through my Strion then I'm going to be needing light for an extended period so the LED on the M6 is great. As well, it can hold it's own for throw (not the best, but adequate). Great light for about $60
And as a loaner/backup I have a Streamlight Stinger XT in the duty bag. This is a rechargeable with some good throw/power. Rechargeable. Variable focus beam for throw or flood. Nice size can be thrown in pocket when not needed. About $100 Canadian.
So my philosophy is to be prepared for everything with a variety of flashlights. A Surefire 6P could easliy replace the Strion on my belt, but with primary 123 batteries, it's a little more costly to run, however a dependable light.
The primary concern with any duty light is dependability and reliability. It cannot fail at the wrong time.