Hi Theo,
Welcome to the forums mate.
So far people who have asked that question seem to always get the same answer: Practice both!!!! the more weapons you have in your arsenal the better. :)
I use Pebbers daily routine as my practice schedule, the picking excercises are meant to be practiced with all possible permutations.
e.g:
u,u,u,u,u,u
d.d.d.d.d.d
u,d,u,d,u,u
d,u,d,u,d,u
+economy picking
It came to me as a surprise (noob that i was) that starting with a downstroke or an upstroke is actually a whole different excercise, and just because you can do one , doesnt imply you can do the other exactly as good.
Check out the pdf files on the site, its all there.
Nevertheless lets wait for what the pros have to say :)