New crankset. There's the key:

While there are other things that could become factors, the single MOST important factor in choosing a bottom bracket spindle length is the crank that you are planning to install. Usually you can get a recommendation from the crankset vendor. Even then, however, sometimes they'll throw you a curve and recommend something like 113/118. If they do that, check out your bike's rear dropout spacing. Use the shorter spindle length for 130mm dropouts and the wider length for 135mm dropouts to sweeten your chainline.