On the a9 possibility: if they have a native mount solution, then you can get 20fps, and that sounds pretty cool. If it is through an adapter, the AF slows and you can only shoot 10 fps.
That said, I think I'd have this lens over the Canon simply for price with roughly equal quality. I will wait on thinking about a9s and the like, though, until they work out a few things, including giving me the ability to use my glass with zippy AF and fps.
-tig
PS: I'm a believer that mirrorless versus mirrored cameras don't matter much at all. It's all about performance, and I don't care what mechanism is inside. Mirrorless makes sense to me for the future, but I'm not going to switch until it is at least as good in the viewfinder and equivalent in other key performance measures, such as AF and frames per second. To my mind that means the a9 is probably the camera just before this inflection point is reached.