Spyder7answered grow question 4 years ago You want negative pressure as that's what keeps the odor in the tent. You don't normally go down in fan size, so I'm not sure if that's a typo or not; either way, I would keep whichever you have as either will do, and is doing what its supposed to.
Being able to run it on low speed with lots of negative pressure is also a good thing, as that means low noise and no fan wear on the bearings in it. Opening intake vents will reduce negative pressure since they increase the amount of air that can come in.