Ah yes, the old in or out debate. This has spawned more than one flame war on the usenet newsgroups. Here's my theory on it:
Unless you have a LOT of lighting in your hood that will produce a ton of heat, I find that one fan blowing in, with the other hole just left open with no fan mounted in it at all, works the best. If you do use two, I would vote for them both blowing in. But, like the man says, there are many different schools of thought on this, and they all will work to some degree. The above is just my personal opinion.
Larry