I own both, and like them both very much, but they're so wildly different, it's hard to compare or decide between them.
L4 - an amazing amount of light for a teeny light - very nice, very white, great for lighting up the whole backyard with a diffuse light. I carry mine in my daily carry bag, and use it regularly.
Z3 - I describe this as 'the brightest light I can shove into a pocket'. I actually end up using it more than my M6, because with the P91, it's darn near as bright as the M6 LOLA, and in a MUCH smaller package. I keep the M6 near my bed as my 'bump in the night' light, but the Z3 lives in the guest bedroom for when I'm in there because the kids are sleeping with my wife (sick, etc.).
The 'spot' nature of the Z3's beam is remarkable, and it's so much more focused than the L4, that it's hard to compare.
That's a long way of saying 'L4 for daily carry, Z3 for occasional semi-pocketable power, M6 for when you know you'll need it, and can put it someplace besides your pocket.'