Is it really just two options? 5 years ago people would have said to use MF (even today, I know someone who at least has access to a MF system), I think today we also have a few native Sony systems, OM-D (I haven't used side by side with a 5D3, but it does seem to have very good shadow noise at the lower ISOs), etc. If you truly live and die by DR, aren't there more reasonable options today than just Nikon?
Not really. For landscapes, you need to have the lens to go along with the sensor. The OMD is quite intriguing, but the lens are not there and the mpix is not there for large prints.
In fact I am holding off from going Nikon as it's TSE lens are subpar to what is available for Canon.