The best answer to that is personal preference. Some people like
20" uni's, others like 24". For stunts and manuverability, the 20" uni's
are a bit more agile. It's easier to spin them around, hop, and do other
fun stuff. This can be an advantage in hockey as well--being able to move
to the puck is a good thing, the faster the better.
Well, and then there are the 24" uni's. If the puck is 20 yards
away, and either you or the guy on the other team right next to you is going
to get it. The race toward the puck will probably be won by the guy with a
24". The wheel size does have a vast impact on speed.
For transportation, about a dozen people here (at Mudd) ride to
class on a daily basis; we all use 24". They are a bit faster, and for long
distance, quite a bit more comfortable than the 20" version.
- John Larkin