the typical issue with crank length on road-to-fix conversions is pedal strike during cornering. a shorter crank (like 165mm) buys you quite a bit of extra lean without worrying about hitting bottom. and hitting bottom is uncool.

as for riding two different lengths, i'm pretty unsophisticated but i've noticed no difference in general pedaling between the 165s on my track bike, the 170s on my road bike (which i haven't ridden much lately) and the 170s on my other fix. i will be swapping the 170s for 165s on the other fix soon, for peace of mind.