A
rubber Diabolo is always the better choice, it's less likely to break
and it's far more stable when spinnng. It also hurts less if it hits you
on the head.
Look
for a Diabolo with some weight as it will spin faster, have more control
and again be more stable when spinning.
You
want a Diabolo with a smooth axel as it reduces the friction and makes
it spin faster and for longer