well you must have nerves of steel to teach kids either one. i'm kinda jumpy myself, so i haven't been around a bunch of kids with tops, but i would have guessed as missile hazards go, tops would be more dangerous. not to mention your kids boring holes through their hands.
at any rate, my hat is off to you for your success in the school yard. i was never the biggest kid, so maybe that's why i have so many yo-yos now.... compensation of sorts...