Getting in a little late on this thread, but here's my .02
If the light is not going to get used regularly, get the 6/9 P or Z series, whichever you prefer. If you already have one and want a little more light at the cost of a shorter battery life, upgrade the to a P61 or P91 bulb.
If you use the light regularly and will keep it charged - go w/the Surefire 8X or 9N. 110 and 140 lumens in a very small package. The charge will drain over time if the light just sits so if this is you, go w/a lithium powered system.
You can upgrade your 6 volt Surefires (handheld and weapon lights) to 9 volts with a spacer.
I use a 6Z w/a P60 bulb, 8X, 12Z (recently changed from the M3) and a 9 volt system w/P91 bulb on an 870P. I've used these awhile so if you have any ?'s on run time, battery life, brightness etc. I'll tell you what I can