It's really personal preference.

My legs are very sensitive to any changes in fit, down to mere millimeters. I had a really hard time finding the right saddle position when I got my first bike. After much agonizing over fit, the LBS noticed that the cranks were 175mm. My bike's specs were for 172.5mm. So, since the bike was built wrong from the factory, they swapped out the cranks. Any knee strain or discomfort I had been having was gone.

I know plenty of people who absolutely do not notice a 2.5mm difference between cranks.

Aside from comfort issues, longer cranks will help you apply more torque to the cranks (one reason some use longer cranks for TTs).