Which is better, a momentary tail cap like Surefire 6P and G2, or a forward clicky I personally like both, but my feeling is that a momentary has less to go wrong, because you tighten the tail to make it stay on.
Most forward clicky switches are also momentary, so you get the best of both worlds. That means they are like the switch on a Maglite. Slight press for momentary, harder press and "click" for constant on.
The momentary switch does have less breakage potential but if a clickie is well designed and made the difference is minimal if you need constant light and only have one hand it is easier with a clickie you can twist most momentaries with one hand though I have a bit a nerve damage in my left hand I have problems gripping and making twisting motions with my left hand so I use clickies my surefires need the occasional drop of lube and they work fine. if utmost reliability and minimal maitenence is your goal go with the momentary. I like tinkering with mine.
Momentary twist has less to go wrong, usually has lock out tailcap and is quiet, but the downside is unless you use 2 hands or are talented you have to release the momentary on and then twist for constant on, where forward clickies you just push down more.
think of f.e. a flupic driven light that needs 2 short on to come to the high as third level
(yes I know there are other MOs also, but I like it that way)
its nearly impossible to twist the momentary every time the light gets used in one of the several levels/modes without using the 2nd hand (at least its boring to do it, when a forward clickie can do this much better)