well on most laptops i beleve there is only one fan on the back, it runs all the time, and usually makes a negligable amount of noise, but still notice able. Most would prolly make the same amount of noise as a standard desktop fan.
If you want to go for quietness then the desktop would be better because you can have more fans running slower keeping the system cool with less sound pollution.
Now if you want silence -=- GO FOR WATER COOLED -=- heh heh