Hotly debated topic here, you may want to try a search.

Consensus runs that if in doubt, err on the side of shorter cranks for the sake of your knees. I've got a 34" inseam and run 175mm and feel that I'm right on the cusp of them being too long for the amount of knee flex I get at the top of the pedal stroke.

Power benefits of +/- 2.5mm probably not as important form the point of view of torque (BTW smaller radius=less torque) as from how efficiently the crank length utilizes your bigger muscles.