I've sat behind various versions of the Rotax 912. Whilst the iS is a technological step forward, I'm not sure it is true to the relative simplicity of the 912 design and do you really need to sip even less fuel than can be achieved by the carb version? Personally, I think the best engine is the plain old 912 as it runs more smoothly than the 912S. I would go for this one unless you feel that you need the extra 20 or so horses.