I'd suggest a 9V unit for a weapon mounted light. I have a 9P and a G2, and I use the G2 for my gopher flashlight. The 9P is very solid and well built and when I get my rail system on my AR I'll mount my 9P with hopefully a P91 bulb which will give me 200L for 20 minutes. These are both pressure buttons tho, not a pressure switch or a on/off. Deciding what type of operation select you want on your light will definatley be a factor in the price. There are some commercially available tape switches for the SureFire lights. The 6P, G2, G3 and 9P all have the same end cap so any pressure switch designed for the 6P will work with anything. However I have heard reports of the tacstars getting hot after a few minutes of usage, and not turning the light on fully.
+1 on Daves.