I think it is a matter of finding your sweet spot. With everything else being equal, the longer crank arms means a slower cadence, but longer and greater stretch of all the tendons going over your knees with each rev. Shorter cranks have your quads firing faster with less recovery time. I ride with a 170mm but at 6’1” should probably have longer cranks if I was going by the book. For me it is not something I would notice much on a day ride, but a few weeks out, riding most every day for six plus hours I think 170 is the right length for me. It is how you feel riding for the last hour of the day and getting back on your loaded bike in the morning that counts. Are you quads shot or your knees blowing up? If the answer is no to both, don’t worry about it.