DrCR's post covers pretty much everything on the current state of the art on fans. Thanks again for his patience and persistence in compiling all that stuff and goading the threads into producing good info.
Just to summarize my own experiences of testing a LOT of 120mm fans. I put the best ones on a BIP II radiator and measured airflow vs rpm and relative noise vs rpm. Undervolting is assumed, and was adjusted to give best performance.
1) There is no quiet 120x38 mm DC fan. They all have considerable motor noise and moderate bearing noise.
2) In the 120x25mm fan category the Nexus 120 is the best. A close second goes to the Globe S1202512L-3M and the SilentX 120. Both have slightly more motor noise than the Nexus. The Globe can push more CFM at max voltage. The Silentx has the dubious distinction of being one of the most expensive fans on the market (4X the Globe) and their claimed noise figures are blatant marketing lies. However it is comparable to the Globe in airflow vs noise. It appears to be a remarked Adda fan.
3) If you need more airflow than the Nexus 120 will provide at 12V you are not going to get a quiet fan as the airflow noise becomes dominant. Get a Globe and adjust to your needs.
4) The 120x25mm fans CFM drops off considerably more than the 120x38mm fans when dealing with the backpressure of a radiator. However they STILL end up having a better CFM vs Noise ratio than the available 120x38mm fans.
Bottom line - If you want the quietest system buy a Nexus 120. If you're not fanatical about the noise but just want quiet, get the cheaper Globe which has an upper CFM around 50% more than the Nexus. In all cases, adjust the fan voltage to achieve the results you want.
For the Fanatics - you can get the absolute best CFM vs noise out of a Sunon A2123 230VAC fan undervolted, but you have to deal with the AC end of it. See this thread
http://forums.silentpcreview.com/viewtopic.php?t=17002