Just from what others have said the White Bros is the fork of choice for TD racers.
Not that I was doing TD but I chose the White fork and found it to be very high quality.
Thanks AZTtripper...Any idea as to why they chose the White Bros? Have you tried any other carbon forks for comparison? I've only ever ridden steel forks for the above mentioned purposes so I don't really have a carbon fork frame of reference.
There are a couple threads over on that other forum that I've read and most would argue in favour of the Niner over the WB. Most were talking singletrack applications though.