Always have more input than output for the simple reason that you need to push air through your PSU. Sure your PSU has a fan but you compromise it's efficiency by having a negative case pressure, that’s why I’d always recommend positive case pressure.
Sure it leads to more dust inside, but a quick blast of air from a compressor/ air gun/ pressurised air can like once a month or even less will sort that out.
I could always have missed some other blinding revelation as to why negative pressure is better and if i have then somebody please correct me.
I agree with GePMan. but on top of the fan in you power supply being less effective if it's got to fight to pull air out, the same would happen with your video card if it's a dual slot one blowing out like alot of the 8800s or ati 3870. it'll slow the airflow and compromise the cooling of your card.
Balanced is the best
inaddition to PSU, as pointed out, negative means few molecules to remove heat from CPU and GPU. Positive pressure normally increases the temp inside the case. So neg and positive pressure has the same overall effec, but for different reasons.
Generally negative pressure increases cooling performance. endorphines and GeOMan are talking about VERY negative pressure. RetiredChief is the closest, you want close to balanced, though slightly negative is the best, very simple example: (all same fans) 5 exhaust and 4 intake. Positive pressure does not cool better than negative, but too much of both is the worst of all. I find that the 900 with the extra side fan is still slightly negative so it cools very well.