I've only listened to the two headphones on separate occasions and not on the same equipment so take these opinions for what they are, opinions :P
The Mad Dog is a pretty neutral headphone, slightly better instrument separation and cleaner bass. The W1000X has a more engaging midrange (or people would say it's peaky in the upper mids) especially for vocals but the resonance adds a bit of coloration that some people don't enjoy. W1000X has a slightly wider soundstage, with a heavy and slightly slow bass response. I've modded the W1000X to improve the speed of the bass a little





























As usual, I would never suggest using graphs as predictive crystal balls.