Pretty much as said above, with an E2e or E1e, you can put on a KL1 or KL4 (even a L2 head, tried this for like a second just to see if it works.) With a C/P/G series of lights with the Pxx lights, you can options too, but give up size. For a lot of dough, I remember this_is_nascar with a P91 cut down and modded E2e with cell extender to basically make a P91 only a little longer than an E2e.
I got a UBH (universal battery holder form Dspeck) with a Z57 tailcap a few days ago that can hold 3 X CR123's with a Z57 tailcap. I am planning on getting a E2C with a Surefire KT-2 turbo head with N2 lamp down the line so I can carry a smaller light (when compared to the SF M6) and still have semi decent output (because I see myself comparing everything I have to my M6 which is really bright for its size, scared a few friends that thought I had one of those spotlights, but were amazed at the small size of the M6.) As UVvis said, like sticking a straw into an orange, sort of like the M4. I don't mind, I plan on sticking this straw in an orange in my hand carry bag next to my laptop.
And the better answer would be get all of them and find one you like /ubbthreads/images/graemlins/smile.gif