Now we see through a glass, darkly...
- Apr 5, 2016
I'd guess that the 70MP sensor was the one they were going to put in the 5DS/R MkII and the 83MP one is a new sensor from the same fabrication line as the new 90D and M6 MkII line.
If they go with the 70MP it will be to claw back some of the R&D spent on that singular use sensor and the RS MkI will have a logical 2 year upgrade cycle and have annoying feature absences. If they are brave and forward looking it will be off the new fabrication line and be the 83MP sensor that will be used for years and will push back a MkII, but it will cost more.
Knowing Canon's business history I'd bet on a 70MP version but I'd like to be proven wrong.
Or maybe they could use the fabrication for the 70 MP FF sensor to make a 27 MP APS-C sensor? Possible uses would be for future Rebels or maybe even a "fast" sports/action camera with either EF-M of RF mount?