My friend and I got an assignment to shoot in a club for their opening ceremony,He use Nikon D800 while I use Canon 5dMark III.I am the one will be processing his and my image once done the shooting.In such situation,I find no benefit D800 towards 5Dmark III.
1st,At high ISO above ISO800 of D800 do not give dynamic range advantage towards 5Dmark III.
2nd,The big megapixel of D800 is a drawn back in rough noise at high ISO.5dmark III(6D as well I belief) i can comfortable to shoot at ISO 12800.
3rd,The big file size is pain in post processing(Extra money to be spend upgrading my PC)
In my opinion,that is depends you shoot.If you are a landscape and studio photographer,D800 is the best choice.If you always have to bump high ISO above iso800 such as event & wedding,5dmark III or 6D is the best choice for the price(1dx if you are that rich).