im 5'8, short legs tho and i have 170's on my old MTB hardtail and 172.5's on my new MTB and roadie. the 172.5's make a massive difference in feel as far as being able to to power down.

it feels quite foreign now when i rdie the 170's. one thing that ive been curious about is willt he 170's given they have a shorter, more compact rotation help improve pedal stroke? i osmetimes feel that my pedal stroke is improved after using them

Totaly off topic, WTF is a compact crank? is it where you run a smaller inner ring or inner / outer ring on the front?