In landscape is probably where the DR matters the most. The D800 seems like the no brainer decision right now for a landscape photographer, especially if you are going to print huge. Do you shoot HDR often? That pretty much eliminates any difference anyway, as shooting HDR means that you can span a much greater DR than either camera.
DR is pretty much the only area in which I would expect the 5100 to be "better" than the 6D. DXO's measurements seem odd to me, and at odds with a lot of other review sites. The rate the d600 as having higher ISO performance than the 6D and in head to head comparisons I just don't see it. Look at some of the ISO video reviews and the 6D is dominating there, for instance. It's just confusing.
I can't imagine the 5100 winning in low ISO noise either, honestly, other than when pushing shadows which is a function of the dynamic range difference.
Not sure this helps, just some thoughts.
|High Altitude Rocky Mountain Railroad by cjf2|
from On the Rails...
|Evening at the lake. by Murat ÜNSAL|