Further comparison data is coming in:
http://forums.dpreview.com/forums/re...ssage=42533115 6D image samples blotchy colour noise
http://forums.dpreview.com/forums/re...ssage=42535983 DXO Scores D600 2nd highest sensor rating just behind Nikon D800.
The question of whether the 6D is a reasonable upgrade from 5D2 though is not answered yet. I suspect it will be based on low noise at high ISO performance and an improved photosite design. Plus the low light autofocus performance of the one cross type central focus point.
It may give a nice upgrade path to 5D2 users who don't need the extra functionality of the 5D3 but want the improved low light performance and don't want to switch brands.
Greg.