Nikon D800 for resolution and higher dynamic range (studio work and landscapes etc.)
Biggest plus is the ability to pullout the details from RAW files. But file sizes are too big and eat up memory and storage space plus will be slow on processing images in PS or LR.
Canon 5D Mark III for high ISO low light performance and sports/wildlife high fps photography. Dynamic range is more or less same as 5D Mark II.
I do not agree with high ISO results of DXO as they are not in agreement with real life shots available all over the internet which clearly show the advantage of 5D Mark III in low light.
Even when RAW files are downsized from D800 they still have little more noise than 5D MarkIII though upto ISO 1600, D800 has more detail but above that Mark III is better.
But the most important factor should be the lens collection that will follow the purchase of a full frame body. Canon has a clear cut advantage in terms of variety and price.
Besides, plz check the new Tamron 24-70mm f2.8 if u r going for full frame.
The best way to judge is to download RAW files from numerous review websites and process them yourself. I did the same with both 5D Mark III and D800 and above opinion is based on my own processing of those files.