Fixed-gear vs single-speed for clipless pedals
Track standing at intersections is more about showing off than about being quick off the stop line. By the time you can track stand consistently (btw, you can learn to do this on a freewheel bike, too), you will have mastered clipless pedals and you'll be able to clip and unclip very quickly. Other than that, riding fixed vs. SS is a matter of taste. Do you like to coast down hills?