I vote for the Canon 70-200/4L myself.
This lens will be MUCH more satisfying for you in the long run, and if you
need the extra reach, the 1.4x Canon teleconverter will make it a 105-280
lens.
The money you "save" from the Sigma will be spent when you decide to repair
because the barrel will decide to get loose or something like that, after
2-3 years of use (I was owner of the previous Sigma version, 70-300 APO
Macro Super, and the difference with "L" glass is incredible!).
The build quality of the Canon is excellent, and it will stay with you for a
LONG time, even if you decide to change your camera body for a better one
(heck, a friend shoot with the 1Ds and the 70-200/4 and he's satisfied with
the image and build quality, plus the weight isn't bad).
If you want 400mm, look at the Canon 400mm/5.6L, which is a very capable
lens as well.
N.F.
(owner of the 70-200/2.8L IS, so I know what can "L" glass do

)