Mostly it seem to be personal preference and what you are used to. Some riders with joint or mobility problems (e.g. Bill) prefer shorter cranks for their smaller circle diameter and lesser vertical travel.

Several years ago Lennard Zinn did an extensive study putting riders of a great range of heights on a variety of crank arm lengths far greater than the 165 mm to 180 mm commonly available. He concluded crank arm length has almost no bearing on power output and was only a comfort factor. The GCN video above seem to support that conclusion. Recommendation: ride what's comfortable.