Shorter crank arms = more fun & endurance
"I pretty much want the fastest speed that I can comfortably maintain. I'm NOT fast, so going from 10 MPH to 11 MPH is a 10% increase! The shorter cranks HAVE helped my speed slightly, but they have REALLY helped my endurance. At this early time, it appears I can ride about 60-70% further with the same level of fatigue. On my shorter errands (1-2 miles), I can "crank it up" an extra 2-3 MPH."
...I agree that short cranks give you more endurance. My speed on flat roads averages 14 mph, but seniors like me can keep it up "forever".