Well the more you undervolt fans, the less difference you actually see between them.
Assuming none of the fans have motor noises, if you run any fan at 5v it isn't really going to be too audible, and due to the low RPMs, you won't actually see much difference in temperature at all between say an AP121 at 5v, and a Noctua NF-P12 for example.
Motor noise is the biggest issue I find really with fans. Coolermaster's motors make horrible noise, which is made more apparent when they're undervolted, and if used on a fan controller, makes them completely unusable IMO. I've heard some issues about the Corsairs making a bit of noise under fan controllers, but they seem fine under straight power.
The only other thing to be weary of is the pitch that the fans make. Some fans like Scythe GTs make a higher pitched noise than other fans, so while they aren't necessarily louder than other fans, they do stand out more.
I do find things incredibly subjective with fans because of this though. So many reviews recommend fans that turn out to produce ridiculous amounts of noise IMO, and there's not really any clear way to objectively measure them without trying loads yourself.
I do see myself as quite a bit of a noise freak, and I have tried a lot of fans myself - just unfortunately for you, not these new German ones that seem to be popping up around the place. They're often too hard to find in the UK.
