yeah, but torque is the turning moment that is applied to the BB axle, and is the product of the force and the perpendicular distance away from the pivot. So longer cranks make it easier to pedal, but require more foot movement to make the rear wheel travel the distance. Its up to you really, 170s, seem too long for mod as they get in the way, but say your sidehopping, you dont need the rear wheel to rotate a lot, but you need the power, so a slightly longer crank is better. Thats why i went for 165 cranks last time i bought some, but not many companies make them that length. 
 
But if you want some cranks, buy my 165 Try-alls!  
 
Andrew