You should select a crank length according to your femur length, i.e., the 175mm cranks are generally for taller cyclists. The longer cranks will give you 3 percent more leverage, but also 6 percent more knee flex and a 3-percent longer pedaling circle. In rare cases, the longer cranks may cause overlap between your toes and your front wheel, but this is extremely rare on today's mountain bikes.
Interesting post above regarding 18-speed half-step plus grannie gearing -- my favourite road gearing scheme is a very similar 48-45-34 / 13-15-17-19-21-23, which works great with a SunTour Cyclone short-cage rear and a Shimano 600 front.