Having used the 7D2, and recently acquired the 1Dx, I believe that having two different slots is a huge mistake. I appreciate the 2nd slot in the 7D, but avoid using it for the inconvenience of dissimilar card sizes and readers.
My workflow is not typical, and I probably use less cards than someone working full time. However, my investment in CF still goes back to my original 20D, then 7D and 7D2.
I will most likely never move to the Mark II, but if I did, I would trust it to to have two identical slots (providing redundancy, and eliminating the opportunity for error). I would expect it to have the fastest capability of its generation, rather than supporting a legacy standard.
While a CR3 may be 'set in stone', I would imagine that, even if it were not truly modular, the opportunity to switch the card modules during manufacturing would not prove insurmountable. This poll provides the necessary impetus for an informed manufacturer to think again...