The Leopard is easier to drive. It has better power on the bottom and top end with the Rotax being very strong in the mid-range. They require fairly different driving styles. Leopard is more point-and-shoot and Rotax is all about keeping momentum and rolling into the throttle smoothly to keep the engine from bogging down.
The Rotax is a bit more bulletproof but requires little tweaks in between sessions to get the jetting correct whereas the Leopard carb can be adjusted on the track and is easier to get dialed in.
The Leopard, though maybe slightly less reliable or durable than the Rotax, requires almost no attention during the day. Slight carb tweaks on the needles is about all you need to do. Personally, I’ve found it to be more enjoyable running a Leopard all day rather than a Rotax but that’s just my opinion. Rotax is obviously very successful so it does a lot of things right.