well.. most of em are jsut expensive names to make a product look cool I gues. if you have a good quality fan, I gues it doenst really matter weither its ball or sleeve bearing. if you have a cheap fan, then my suggestion would be sleeve bearings, because ball bearings are more expensive to make, I would gues the ball bearings in cheaper fans.. well, I'm not sure if they'd last long.
In my case I'm using Antec Tri cool fans for about 1,5 years now, and they are still working great so I can only recommend those ^_^
Isn't that loud compared to the cheap models that sells for 4$, if you want ultra silent i would go with something like this but remember ultra silent comes with a price to pay