I have a U2, it's a great light, but recently I use my HDS U60 more -- it's modified by Milkyspit with an SSC emitter for about 120 lumens, about equal to a Novatac 120P.
My U2 has a Leef body so uses an 18650 cell, which is great because it's rechargeable. But my modified U60 has roughly the same output, albeit for a shorter run time. But mostly you don't need full output all the time.
The smaller size of the HDS/Novatac, more output levels, and much dimmer low level (a U2 weakness) are advantages.
They are both superb lights, but the U2 is expensive, and if you want 18650 capability that adds yet more cost.