I EDC the U2 and LOVE it.
I also have an Arc4 (very similar to HDS) and like it also,
But I really like the output of the U2 better,
The HDS probably has alittle tighter beam, but the U@ is alot more light, and makes up for that.
Plus with the U2 you have all 6 levels handy at all times.
You arent forced to select only a couploe levels as your primary and secondary levels.
The U2 can also use Bare UNprotected 18650 cells,
which is a VERY nice reason to EDC it, especially if you will use it alot.
I hate using primary 123 cells, so I wouldnt like EDCing the HDS for heavy use.
Ruggedness?
eh, they should be about the same, though I would lean slightly toward the U2.
Whiteness?
Well thats gonna vary, due to the luxeon lotto.
I think though, on average the HDS might be whiter.
Then theres the size differene.
the U2 is alot larger, which may bother you as an EDC, though I dont mind it in my pocket at all.
I do find the U2s larger size , and shape alot easier to handle and use.
Heres my U2 review.
http://www.jtice.com/review/lights/u2/
~john