The C3 lens is lexan, like the 9P. I managed to bubble the lens on my 9P after about 5 minutes of constant on with the P91 bulb. At the time it didn't really bother me since when the P91 bulb was introduced Surefire warned that it was intended for intermittent use only and that you could melt your lens with constant running. I've noticed that they no longer point this out on their web site.
For me, the regular P90 bulb never seemed like it was enough brighter than the P60 to justify the 50% increase in battery cost, and due to the larger filament and longer bulb, every P90 lamp I've seen has a more oval, egg-shaped beam than the P60. Admittedly, the P91 is very impressive in terms of output - the beam may be oval, but it's like holding a portable video light. Unfortunately, you pay for this in terms of battery life. It's actually worse than advertised - while yes, the P91 will run for 20 minutes, it draws so much current that it is noticably yellower after 5 minutes or so and usually after 7-10 minutes you will get MORE light from the partially depleted batteries if you swap it out for a regular P90.
After the lens-melting incident I purchased one of the old 3" turboheads for my 9P, and, frankly, this is the way to go. "Only" 105 lumens, but you now have a perfectly round beam with an honest 100-yard hotspot, reasonable battery life, and none of the "afterglow" you get with the high-current lamps like the P91/61.
I'll stick with my 9PT, but if I was buying new I would probably just go for the M3T or, more likely, the M4, to get the extra bulb and increased durability. As somebody else pointed out in one of these forums, if you look at lumens-per-battery-per-minute, the M4 is the top performer in the Surefire line(225 lumens, 4 cells, one hour).
-BCK
