In reply to The Lemming:
It's not really such a distinct 'us and them' gap. They've both advantages and disadvantages. Mirrorless will probably be the future but it's not quite there yet. There's lots of rumours that both Canon and Nikon will be producing full frame mirrorless cameras.
I'd worry far more about the lenses.
The biggest mirrorless contender to the pro / semi pro (medium format aside) Canon and Nikon DSLR's is the Sony Alpha a7 series. But whilst image quality is great they have a limited range of lenses particularly at the extreme telephoto end. The ergonomics are not as good as the Nikon D810 / Canon 5D IV. The battery life isn't as good and it's not as robust. The lighting systems are getting there but still don't really compete when you want to sync multiple speed lights and mono lights from various companies together.
The size and weight advantage of the a7 series is negated completely when you put a pro zoom on it (the 24-70 and 70-200 are both bigger than Canon / Nikon counterparts). Finally the Sony GM series is as, if not more expensive than the Canon / Nikon equivalents.
The more competition the better though and it's an exciting time for camera innovation.