Are you a spinner or a masher? I would presume that spinners would prefer a shorter crank and mashers a longer one. Your height, leg length and frame size are other relevant factors. In my case, all of my bikes have 172.5 cranks. When I buy a new crank, I search until I can find a 172.5. My knees are very susceptible to small changes in saddle height, so I would rather not mess with a good thing. I know that 172.5 fits me just right, so why change? However, 172.5 cranks are getting harder and harder to find as manufacturers limit their product lines.