Punx0r said:Saying "less unsprung mass is better, therefore mid-drive is better" is a gross over-simplification. You can't judge the overall suitability of a system based on one axiom.
On technical subjects some people know what they're talking about and some people think they know what they're talking about. The latter tend to draw sweeping conclusions based on a single factor, while the former have the knowledge and experience to arrive at the best compromise that works in a real-world application. This is usually non-ideal in most or all factors. That's what you pay an expert for: google will tell any idiot in 10 seconds that "unsprung mass = bad for handling", the expert can tell you whether or not this is relevant in a given application.
Bumpy off road riding is near the top of the list of applications where you'll benefit from less unsprung weight. Here's a page put together by an "expert" who does motocross suspension: http://www.mb1suspension.com/#!unsprung-weight/c15fs In particular:
MX site said:Factory teams spend BIG BUCKS on lessening unsprung weight. Most works bikes have magnesium hubs, titanium linkage bolts, Ti axles, aluminum spoke nipples, tapered and butted spokes, titanium shock springs, carbon fiber chain guides and magnesium (or beryllium) brake calipers.
Anyway, you don't need to be an expert to understand that, just have some riding experience and spend 20 seconds thinking about a wheel interfacing the dirt. Since the wheel and attached mass are forced to contour directly over the ground they'll be able to do a smoother and more consistent job with less mass. Anyway, even on relatively smooth tarmac tracks race teams have been putting huge effort into making things like carbon wheels and aluminum sprockets.