Just in case you were considering the 5D mark iii and the Nikon D800 as your choice this link may be useful:
http://forums.dpreview.com/forums/re...hread=40909413
It puts to rest the notion the 5D mark iii will outperform the D800 in low light high ISO RAW images. Canon has a more aggressive Digic 5 noise reduction that seems to work very well in jpegs at the cost of some miniature details. So jpeg performance seems to favour Canon. If you shoot jpeg a lot of the time that may be a worthwhile consideration.
The D800 has higher dynamic range by 2 stops, higher sensor QE so ISO's don't quite match up in comparisons and also the ability to be downsized to 22mp from 36mp to further reduce noise in jpegs so the 5D mark iii jpeg noise advantage will not be by as much as you would think for those reasons.
So you can have your high megapixel cake and eat it too!
Either way both cameras seem to be sensational.
The real super high performance low light cameras seem to be the 1DX or the Nikon D4. Those 2 cameras are much more evenly matched and if it were my choice I'd pick the 1DX.
The choice may be more determined by how much money you have invested in lenses.
Greg.