Good question, answer: every application has it requirements. In the case of a Q-40 vibration must be considered, so a good quality servo is a must.

As for speed my opinion says faster is better. When I first started racing just staying in the sky and not killing my competitors were high on my list. Now that I can scribe the corner pretty tight I can sense the difference in servo speed when going from one end of servo travel to the other. When I fly my old scat cat with Futaba 148 servos I learned to let off of full elevator going around turn 1 before it really did, taking in account the slower servo speed. When I bought my first "fast servos" I could sense the difference and gained confidence knowing I didn't have time my stick inputs around the the fact of waiting for the servo to catch up.

This of course is just my opinion to a very good question.

We need a world RCU pylon championship race. That would be sweet...