Originally Posted by
pierce
those are both 170mm long arms, which is the 'standard' size. you might look on the back of your current crank arms and see if there's a number like 165, 170, 172.5 or 175 (those are the common crank arm sizes). longer arm gives you a little more leverage, but means your knees travel farther up/down. its all a tradeoff. short legs prefer shorter crankarms. shorter crank arms are easier to 'spin' (pedal at high cadence), while longer arms tend to promote 'mashing' (pedaling with high force).
The difference in crank length between 165 and 175mm is obviously 10mm, or as about 6%, in other words very little. I've always suspected that any actual difference in "feel" between crank lengths in between those two values is mainly down to expectation - a rider expects them to feel different, so they do.