I can't compare directly the 2 fans as I never had them together at the same time...
But I have a Delta fan (the loud one) sitting on my Hedgehog which sits on my Duron.
The first time I powered on the fan outside the case, The lounge room was filled with the awe and power of this fan - which means it was loud. My g/f in my room some 10 metres away even heard the fan power on.
I have a LianLi Aluminium case (not sure if this cancels noice better than conventional steel and plastic cases). I have 3 case fans, 2 front, 1 back. Also 2 fans blowing in the power supply (Enermax). I have a 40mmx10mm fan sitting on my GeForce2MX chip. With the Delta fan running, it is not at all unbearable with case closed completely. I even let the computer run SETI all night long while I sleep in the same room. No nightmares of big fans chopping me...
So, my point is that if you are afraid the Delta fan is too loud, IMHO, it should be ok as long as you keep your case closed.
To answer your question, the Delta fan has a high pitch hum which is rather distinct due to the high speed of the air flow. The Sunon fan certainly does not have this high pitch hum.