I have been using this as a reference for some time now...
http://flashlightwiki.com/Cree#XP-GIt gives an overview of the types of Cree so you know what to expect when buying a light. In order of power and efficiency the current hierarchy seems to be...
XM-L T6, < the original led which led the charge of affordable but powerful lights a couple years back
XM-L U2, < current sweet spot of price and power
XM-L2 T6 < not that common but might end up cheaper and more common than than XM-L U2
XM-L U3, < not that common
XM-L2 U2 < current power and output monsters appearing in a few lights
With the wiki info plus knowing the voltage and AH of your battery, you should also be able to work out predicted lumens (not lux) and run time, reasonably accurately. I tend to ignore what the seller says in terms of output and work it out for myself if possible.
Optics and battery management / quality obviously make a big difference to the light too. A good light is a balance of weight, output, size and ease of use..... not just power.
Im no expert .. so just my take.