Rotax seems to have good reliability and easier on the kart as well as smoother. Yeah the Leopard starter is always an issue it seems. I have received some advice that Rotax turnouts @ NJMP are light. I know Leopards are plenty in this area and yes I am a believer the deeper the class the better!! Leopards seem like they do need rebuilds sooner but they are up there in the RPM range so not too much of a surprise.
What about tires, this seems to be a real issue with karting, new rubber is important and @ $225 for a set of 4 seems to add up quickly.