I apparently glazed over the Noctua fans quickly as the specifications listed for them weren't as straightforward as the other fans I listed.
Though, looking at the specs of the NF-R8 on the Noctua site, the air flow is listed as 53 m3/h. According to Invalid Link Removed, that is 31.1958CFM at 17dBa.
The SilenX EFX-08-15T is listed as 32CFM at 15dBa.
You say you've used both and the Noctua is slightly better. Now my question to you is, which of the two companies lied about their specs? Is the Noctua quieter than they say or is the SilenX louder than they claim?
I suppose it's a moot point either way if the Noctua is, in fact, noticeably better. The fan is available for around $19 shipped nearly everywhere.
Invalid Link Removed
Duly noted!