Q: Which is a better crank length for trail riding, 175 or 170?
A: There are various pros and cons to running different length crank arms on a mountain bike. There isn’t a clear “winner,” so rider body type, bike geometry, and style of riding should be the deciding factors. Generally speaking, the longer your upper leg is, the longer your crank needs to be in order to feel comfortable riding and possibly avoid knee strain.
Longer cranks can be an asset for climbing, but a liability as they are more prone to bottom out when pedaling through rocky terrain or cornering. Bikes with a low bottom bracket will probably want to utilize a shorter crank, while high bottom bracket bikes might want the extra leverage of a longer crank to help with climbs.
Factors to consider, when sizing your crank arms, include saddle position (fore/aft) and height, cleat position, and if any of the sizing is coming at a cost of other variables such as crank stiffness or crank weight.
The bottom line is comfort, so if you are in a position to try both sizes, do so, but if this if for a new purchase where you have to make a call, consider your upper leg size and bottom bracket height into the equation.