Canon 5d Mark 3 vs Nikon D800?

Which camera is better for video purposed and which shoots better pictures? How does the lowlight performance compare?

According to this:
http://www.cameralabs.com/reviews/Canon_EOS_5D_Mark_III/Canon_5D3_vs_Nikon_D800_noise.shtml
the Nikon captures better detail than the Canon below ISO 800. Above ISO 800 the Canon suffers less from noise than the Nikon.
This article compares video performance:
http://www.dslrnewsshooter.com/2012/04/01/video-shootout-nikon-d800-vs-canon-5d-mkiii/

For recording video, you should buy an actual camcorder/ video camera. Despite popular belief, DSLRs not not replacements for actual video cameras. DSLRs are designed and meant to shoot stills. On top of the $3, 000 for either of those cameras, you'll need several thousand dollars worth of lenses and thousands of dollars worth of other accessories.

Both can handle HD quality videos. As a Nikon user I would recommend Canon system for videos, Nikon is better for still photos. Nikon D800 is not ideal for sports photography due to the size of each files being a 36 MP camera which could slowdown when camera buffer is used up. If you specifically require DSLR for HD Video production I would recommend lower models, they would be cheaper and they would be able to handle 1080p as well as they will be cheaper to replace should it get damaged. For low light photography, I say D800 since it will have more pixel information but 5D mark III is not too far behind and in some cases their differences might be negligible.