There really is not much of a difference between 170 and 175 mm cranks.

Really long cranks tend to be used for people with really long legs and proportionately longer femur lengths to maintain proper position when a shorter crank would push them farther back and require more setback on the seatpost / saddle.

I ride 170, 172.5, and 175 mm cranks... I spin like a gerbil on crack regardless of the crank length and the change in leverage is pretty negligible, gearing choices make more of a difference.