Full Frame Mirrorless???
are actually going to release one, by all accounts:
Right now I would chose the Sony version , even though I a) have a ton of Canon glass and b) don't like Sony.
Why? For starters they have significantly more experience producing good mirrorless cameras. But it's more to do with my intended use. Right now the Sony sensors are ahead in both resolution, and more importantly low ISO dynamic range.
See, a full frame mirrorless is more than just an expensive bulky point and shoot, it's an affordable mini digital back. Coupled with a sliding adaptor, bellows and lens and you've got a digital view camera. No need to mess about with sync cables and leaf shutters it has it's own FP shutter, no need to manually focus by looking at the ground glass with a loupe, use magnified liveview.
DSLR bodies are too deep to work as digital backs with wide angle lenses, APS-C mirrorless bodies would require too many shots for a pleasant shooting experience. MF backs are too expensive for most. With a 35mm back and 4 shots in a square pattern you can almost get 6x7 coverage at potentially 128 megapixels, you can easily beat the IQ180 for resolution and size.
A mirrorless full frame camera is also a universal 35mm system capable of taking virtually any lens designed for 35mm, including all the Leica M lenses. It's also the ideal landscape camera, weighing much less than DSLR but capable of matching the very best DSLRs for image quality.
But Canon really have to really step up their sensor performance and mirrorless operation/handling (particularly the AF) to stop my money going to Sony instead, compatibility with EF is not enough now there are autofocus NEX-EF adaptors out there.