I read that blowing air out and creating negative pressure in the box is good for the plants. I think that blowing the hot air out may be more effective as well, if the closet is fairly airtight, because you'll be pulling in fresh air at close to the same rate as you're blowing it out.

On the other hand, blowing cool air in, and putting it where you want it sounds pretty good and if you're forcing the hot air out the top...the same principle in reverse.

I'm not sure if one is better than the other. I can say I've read alot about passive intakes.