38mm is better....
I dont have any "stats" to prove it but in general they are better as they have a higher static pressure (pushes more of the air through the true - rather than just moving the air through the fan) etc
If you can get your fans on a Sans Ace 1011... then your TRUE will be Godly. Very noisy, but effective. They cost a lot too. Not sure where you can get them though. They are usually out of stock. Once you see one, you shold catch them. I think someone might be selling them in the forum though.
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