Riders who come from freestyle use 20 inch unicyles for all their coasting.
Riders who come from track racing use 24 inch unicyles.
The difference lies in the inertia of the unicyles. A good coaster can coast until he is almost standing still. But with different wheel sizes the coasting abilities of track coasters and freestyle coasters are not comparable anymore.
Riders who come from freestyle use 20 inch unicyles for all their coasting. Riders who come from track racing use 24 inch unicyles.
The difference lies in the inertia of the unicyles. A good coaster can coast until he is almost standing still. But with different wheel sizes the coasting abilities of track coasters and freestyle coasters are not comparable anymore.