I am not aware of any downsides to the T mount version, but if I was sure I was staying with a particular make of camera then I would go for the fixed mount as I believe that the tolerances would be better than that of a variable mount as it would be specifically made for the camera. But if there was a chance you might want to change systems then the T mount would be a better option.
Thanks a lot for all the info which cleared up my doubts. I'm pretty much locked into Nikon and Hasselblad. Lots of lenses of both and not much of a second hand market. Plus, for tilt-shift you need an FF sensor so Nikon it will be.