The Antec High Current Gamer HCG-520M uses an 13.5cm, 2BB, ADDA ADN512MB-A90. This fan, in the non-modular version (which is exactly the same minus the modular interface), was measured spinning at 600RPM in idle and rises to 1800RPM on full load.
I'd say based on the fan curves I've seen that it is quiet up until about 60/70% (300W) load.
Whereas CM claim that the fan speed range of the SPM2 520W is 900-1300RPM, so better in load, slightly worse in idle.
CM claim that it uses a Hydraulic Dynamic bearing (which would be better than 2BB), but according to this TPU review
(assuming the 1000W and 520W model use the same fan) it's not really a HDB fan, but a Hysint fan. Even so this bearing type is quieter than ball/sleeve.
joluk44 might not be right about the CM SPM2 series units, but he is right that being a Seasonic unit doesn't automatically make it better than competitors.