Which is more durable. CPU or FAN?
The MacBook Pro can get quite hot and the fan may often spin at high speed and this is considered normal operation. In addition to it's fan it also dispels heat via its unibody aluminum shell. This can make the surface very hot but this is often a good thing as it is moving heat away form the CPU and other components. As long as the ball bearing does not get clogged with dust then it should last a good long time even when running at high speeds. The MacBooks all include temp sensors which will power the MacBook off if it exceeds the heat tolerances. 

I think the question is not phrased quite right because as long as the fan does not fail and it keeps the CPU cool, then it will last for many, many years. And as long as the ball bearing doesn't wear out or the blades become unbalanced you will get years of service from your fan. Does this help?