Tim,
Alot depends on the type of track. The fast tracks (IMI, GJ) rotax runs great, the tight tracks (Track, Bandi, CRE) Leopards run great. Angel is absolutely right when he talks about chassis set up and driver skill.
I chose a rotax because of the many options I could have in racing venues:
IMI Club
IMI Rotax Challenge
Tag World’s
CSC Tag
CRE club
CRE rotax
Track Club
Track Rotax ????
Also if you maintain it well it will give you little problems. In 8 yrs running it (4 yrs on this engine) 1-major rebuild $800, 1-piston/ring change $125, 2-ring changes $100 ea..