actually for UL backpacking, the Fenix is so much smaller than the U2, i'd go with the Fenix.
My preference is a L1D-CE or a L2D-CE since i'm such a "light hawg" due to both my proclivity for night walking (longer burn times) and my age-degraded low light vision; plus, and here comes a somewhat illogical personal preference - i really like a tactical style butt/tail cap switch and the ease with which modes can be switched. However, for those who rarely have the need to switch modes the L0D-CE and the P1D-CE are both very excellent too.
Clearly, for most the tiny L0D-CE is fine - especially for younger non-smoking trekkers with no medical issues, or on meds which may affect one's low light vision, and who also are engaging in good nutrition - all things which can affect one's low light vision.
I think if one were to just consider any Fenix vs. the U2 just on the basis of features (quality seems comparable) and all around usability, i, personally would give the nod to the U2. For the price, it better come out on top!!
Lux and Lumen outputs can really only tell part of the story. Vision is such an individual thing. You really have to SEE the lights side-by-side to compare them. Drehrer was spot [pun intended] on when he wrote in his recent article that "beam pattern" was most important to him. The Fenix beam pattern is really very, VERY nice, IMO, but the U2 is better, IMO. YMMV.
The price alone is enough to make the U2 a VERY unattractive choice. One might say, and i'd partly agree, that one has to be crazy [never said that i was sane!!] to pay that much for a flashlight, unless one's life depended upon it (law enforcement & military - they have a very definite need), or perhaps if one was a collector (though sanity might still be questioned in this case!).