Both have their strengths in which they are better than their rival. Neither have real weaknesses, though compared to each other, you could say Wout's weakness is his technique, Mathieu doesn't really have any weakness and is the most balanced and allround rider of the bunch. Wout is a better runner, though Mathieu is still an excellent runner. Wout has more power, though Mathieu is not far behind and both are miles ahead of the rest. Wout seems to handle extreme weather conditions better and i also still give him an edge when it comes to fighting spirit and mentality. But Mathieu is so far superior on a technical level, and even if he is somewhat behind Wout on a few other key points, technique is more often than not the deciding factor, especially on a fast course. One sliding, having to put your foot on the ground and get going again, can cost you 3 seconds. A second and third mistake, you might not make up for.
This year we've seen more bad weather than the past few years, hard terrain, fast courses. They favored Mathieu in a way nobody could come close. In bad weather and hard conditions on slow courses, Mathieu becomes human. But even then, probably only one guy can beat him.