Are you concerned about Nikon longterm?

Concerning the obvious; you can just downres the D800 files and they will far outshine all of the other cameras. DXO shows this, it also shows that the D800 at per pixel level isn't particularly fantastic (have a look at the graphs and click 'screen' their data is indeed very accurate), not surprising considering the sheer amount of sensels on that sensor in comparison to other cameras. Those pixels aren't particularly great on their own - but working in unison and correctly processed they are stupendous.
Try downloading the RAW file, downresing it (using Lightroom or anything else worth using) to 20mp and comparing to the 6D. Oh look - it wins.
Now for a laugh try taking the 6D RAW file and upping the resolution to 36mp. Yep.
I am not a Nikon fanboy, this is just data - irrefutable and delicious. I shoot Pentax anyway