Heathpack 

Interestingly, my fitter measures power output while pedaling, maybe you guys do this too? I do output more power on shorter cranks- 165 mm. The bike I eventually purchased came with 170 mm cranks. They work for me, but I get this dead spot of power output at the top of my pedal stroke on the 170 mm cranks. This won't hurt me, but I get a little more power out of the 165 mm cranks. So my fitter told me the pros and cons- basically the only advantage of the longer cranks was no additional outlay of money, the shorter cranks were better but the difference wasn't huge. He let me decide for myself, gave me enough info to make the decision. Showed me exactly what the differences would be.

In the end, of course I got the shorter cranks. I like everything to be "right" on my bike. I bought a back-up bike and it had 170 mm cranks which I wasn't going to replace. Haha that lasted all of two weeks. As an endurance person, any little tiny incremental increase in power output becomes significant after 7 hours on the bike. Plus a huge advantage for me in that I can spin very well, the shorter cranks make it easier for me to have a wider range of useful cadence, another giant plus for me on these epic rides.

Left to my own devices, I would have maybe figured all this out eventually. Or maybe I wouldn't have. Who knows? But as a newby rider, I've gotten to a pretty advanced degree of understanding about some of this stuff. Why? Because I was open to it and willing to pay a very fair price for some real expertise.