I much prefer ball bearings over sleeve bearings. They last much longer and tend to run a little quieter. They do cost more, but I suggest the lifetime of a cheap fan with sleeve bearings is less than half that of a ball bearing fan.
Ball versus 2 ball? Well, any fan actually has two bearings in its motor - one at the front where the shaft emerges from the motor and the fan blades are mounted, and another at the back end of the fan motor. A few fans are made with a sleeve system in one position and a ball system in the other. Their problem is they only last as long as the weaker part. Once the sleeve bearing wears out, it does not matter that the ball bearing is still just fine. So, some makers emphasize that they have NOT done this by specifying 2 ball bearings, or "all" ball bearings.
sosfm makes a good point, too. What type of bearings is only one consideration in choosing a fan. Noise level generated and air flow rate (in CFM) are very important. In truth, the noise generated by the bearing system is quite minor until they wear out, and then you hear squealing. For the most port, noise is generated by the fan blade design. Fortunately, noise can be measured and specified as dB (decibels) at a given speed and airflow, so you can compare in the fan specs.
Have to agree with Davcon those are the bearings that they use in HDD's too.....
The type of bearing used in a fan can affect its performance and noise. Most computer fans use one of the following bearing types:
* Sleeve bearings use two surfaces lubricated with oil or grease as a friction contact. They often use porous sintered sleeves to be self-lubricating, requiring only infrequent maintenance or replacement. Sleeve bearings are less durable at higher temperatures as the contact surfaces wear and the lubricant dries up, eventually leading to failure; however, lifetime is similar at relatively low ambient temperatures. Sleeve bearings may be more likely to fail at higher temperatures, and may perform poorly when mounted in any orientation other than horizontal. The lifespan of a sleeve-bearing fan may be around 40,000 hours at 50 °C. Fans that use sleeve bearings are generally cheaper than fans that use ball bearings, and are quieter at lower speeds early in their life, but can become noisy as they age.
* Rifle bearings are similar to sleeve bearings, but are quieter and have almost as much lifespan as ball bearings. The bearing has a spiral groove in it that pumps fluid from a reservoir. This allows them to be safely mounted with the shaft vertical (unlike sleeve bearings), since the fluid being pumped lubricates the top of the shaft. The pumping also ensures sufficient lubricant on the shaft, reducing noise, and increasing lifespan.
* Ball bearings: Though generally more expensive, ball bearing fans do not suffer the same orientation limitations as sleeve bearing fans, are more durable at higher temperatures, and are quieter than sleeve-bearing fans at higher rotation speeds. The lifespan of a ball bearing fan may be over 60,000 hours at 50 °C.
* Fluid bearings have the advantages of near-silent operation and high life expectancy (comparable to ball bearings), but tend to be the most expensive.
* Magnetic bearings or maglev bearings, in which the fan is repelled from the bearing by magnetism.
Top of the list would be the Cougars since I have seen them work and am really in love with the sound/no sound....
Colors are a little off.... but hey, look at the Noctua ones... this would do anyday....