2x 600w is the way to go. 600w is the efficiency sweet spot. You get disproportionately more heat going from 600w to 1000w. So you can get those 600w bulbs that much closer to the plant. Equally important is the coverage you get from 2 smaller watt bulbs vs. one large bulb. Think about it, with two bulbs you will have considerable light overlap and you will be able to light all corners of your tent. You'll find a link in my signature that will help you with your fan questions. The issue you need to keep your eye on is air leaks around the fixture and any attachment points.