You'll get lots of opinions on this. The only general observation I have on it is that 170mm cranks can work well for people who have a strong bias for reducing pedal smack and like to spin instead of mash. Power pedalers who like a powerful, slower cadence are usually drawn to longer cranks. People are different. Their physiology and even just plain preference can vary greatly. Some think 170mm cranks are only for DH/FR bikes. It's not as black-and-white as that. I'm 6' tall with a 32" inseam and prefer 170mm cranks on my AM/trail bikes. A couple of guys in our riding group have180mm cranks.