I dont know what else youve got bc - but are you trying to decide based on the engine, the handling or the styling ? I've ended up with so many bikes cos they are all different - and I really can never say I have a favorite for more than a week
Do you have access to any small children that you could sell off. If not keep the 851, and wait. The motorbike Gods will get you a 916 if you have been a good little vegimite. Griff