I personally would use the slower fans, just because I'd like to keep it as quiet as possible. As far as fan sizes go that cooler supports two 120s or two 140s so you can grab either, though keep in mind corsair doesnt offer the SP series in 140mm yet, so your only option would be to grab the 120s. They might be only slightly louder, but I doubt it will be massive performance difference between two 120s vs two 140s.