In a similar situation, I was thinking end of last year to switch to a Sony A7r, but not very convinced of the performance of the Metabones adapter with the TS-E 24mm Mark II (it looks like it is a 50% chance that you get a good or bad metabones adapter for what you can read in different forums... and the shops in Barcelona usually they don't stock the adapter to try several there before buying one...).
Right now, I will just wait, see what Canon announces next Friday, it is clear that the Canon 5Ds/5Ds R are thought as studio and landscape cameras, for landscape, DR is like an strong requirement... anyway, I will wait until I see the first reports from different users of the camera or DxO before I put my money on it (also, there is the possibility they use Sony technology on that sensor... although this is a rumor that I don't believe too much...). If the rumors are true, the camera will hit the market around March or April, I suppose we will get the reports/reviews around that time...
Also, if you are thinking to switch to Sony, there is also the rumor Sony will launch the A7r II or A9 (or whatever name they choose), possible that camera will also be a 50 MPx camera... or close enough, and being an exmor sensor, DR will be great...
@Torbjörn thanks for that link... I was not aware of that report...