Another optionis the Streamlight Scorpion. Scorpions are popular with LEO and EMS personnel in our area, primarily because it's less expensive than the Sure-Fire. I have both, and I actially prefer the Scorpion.
They're about the same size as the Sure Fire 6P (barrel is 7/8" vs the Sure-fire's 1.0"), takes the same two 3v lithium battieries, and puts out about the same light as the standard Sure Fire (Sure-Fire does make a brighter front end for the 6P) The Scorpion's barrel is covered with a rubber sleeve for a non-slip grip with sweaty, shaking hands, and its momentary switch is also its permanent on-off switch; just push it in all the way to turn on or off. This takes a little more effort, and is difficult to do accidentally.
The Scorpion's biggest advantage is that it's less likely to be activated unintentionally. My first small tactical light was the Sure Fire 6R, and I learned right off that when the tail cap was adjusted just rightr for easy momentary switch use, it could be activated by SIDEWAYS pressure on the end cap, something that happens frequently when seated. Carrying lens-down further aggravates the problem. I once felt an extremely warm sensation on my hip while in the courtroom, and discovered that I had ruined had my kydex flashlight carried from Arizona Tactical Accessories, and drained the ni-cad battery in short order. Removing the extra barrel section and using 2 3v batteries, a la 6P, the problem was the same. Also, carrying the SureFire lens down, the tail cap tends to either tighten or loosen when getting in or out of a car. Carrying lens up is better, but not much; I went back to my Sure Fire when I got my new combo magazine/Sure Fire pouch from Rafter S, and I had it come onin the pouch. Seems like the sideways pressure of the car seat against the head of the light caused sideways pressure of the tail cap against the pouch, and it activated. (The latest incident AFTER I installed a newer model tail cap that wasn't supposed to let this happen.) My fix for the Sure Fire was to install a thin rubber o-ring jsut forward of the tail cap threads on the barrel, and now the tail cap won't go far enough forward to activate unintentionally, (or at all, for that matter) but the momentary switch will still work. While it won't screw in to activate accidentally, now it also doesn't UNscrew to where the momentary switch won't work, which it also used to do. Since I almost never use the permanent on-off feature, this causes me no trouble.
If you have trouble deciding which one to get, my advice is to get one of each, carry the one you like best as your primary, and keep the other for a spare.