I suppose you want to compare the StreamLight (aka Insight) M-3 Tactical Light and the SureFire Nitrolon Polymer Handgun Tactical WeaponLight (The P1xx)
These two lights are both 2 cell 123A lights, however, I believe the SureFire P1xx has one significant advantage over the M-3, and that is the optional P61 high output Lamp Module.
Compare the options that SureFire offers with those of other brands, and you don't just get a WeaponLight, you get a way of thinking.
Al