I have no real expierance of either but this is my opinion. If your hands off buy a new rotax. Get it serviced. Run in on unleaded. Make sure it starts well to look after the sprag. It will more than likely last the 2000 hrs just about trouble free. If you hands on get a Jab.I'm sure they are both excellent and in my experience a well looked after engine generally tells you before it stops.
Lyndon