Positive pressure (intake) would allow you to filter the air your pulling in. One issue with active ventilation is that you'll be blowing a lot of dust around. If you exhaust, dust will get sucked over the top of your tank and settle.
Do you have holes in the top of your canopy already? Is the fan just for air movement? If so I would definitely say intake from the side across the light. If there are no passive.ventilation holes the best case scenario may be equal parts intake and exhaust. Intake on one side/rear, exhaust on the other (near the top). Or intake from the side/back and exhaust from the top.