40mm's spin faster, but you can turn down their rpms and in this case the main advantage is that you can place them directly by the VRM heatsink for optimal air flow where you want it.
Noise is a product of air movement, vibration and bearing friction.
This looks interesting:
the 40mm variant is rated at 19dB @ 4500rpm. Turn this down and I'm sure it wont be more audible then other fans in your case.
Hopefully I'll soon be able to test mine and it's effect on my VRM temps.
Mine don't run quite that hot (94C in Furmark with 1100/1650 oc), but I want to make some headroom for more OC :-).