Its funny, because the 'pressure' thing isn't proven either way either... about the only thing people can say there is positive keeps some dust out by not taking air in through unfiltered cracks in the case. But remember, pressure is a negligible thing considering a PC case isn't remotely sealed.
Positive Pressure has more Pros than Cons as opposed to Negative Pressure. On top of keeping dust away, your rads enjoy cooler ambient air.
Now with Negative Pressure, where is the cooler air coming from? The cracks? I feel there will be more resistance as it's chocked for air, even more so than going through a rad.