Now this is just some speculation on that from me but..
Standard 25mm thick fans should be able to push similar amounts of air with less noise due to better room for blade shape (Less turbulence) and better motors & bearings.
Not to mention that a single row of fans would mean less turbulence just that and less moving parts in your system!
It would however depend on what fans you'd choose and how much clearance you have as two rows of slim fans could be both a bit thinner and a lot thicker than standard fans and 2x18-20mm thick fans might offer the best performance if 2x25 would be just a tiny bit too thick.
Standard 25mm thick fans should be able to push similar amounts of air with less noise due to better room for blade shape (Less turbulence) and better motors & bearings.
Not to mention that a single row of fans would mean less turbulence just that and less moving parts in your system!
It would however depend on what fans you'd choose and how much clearance you have as two rows of slim fans could be both a bit thinner and a lot thicker than standard fans and 2x18-20mm thick fans might offer the best performance if 2x25 would be just a tiny bit too thick.
