I can speak on this with some authority because I have and regularly use both systems.
Both Nikon and Canon each have their advantages. I shoot all my stills on a Nikon D800 but I shoot most of my video on the Canon 5D MIII. I use only Nikon and Zeiss lenses for both however. I don't use Canon glass at all. I much prefer the Nikon glass.
For the average consumer either will work great and take amazing pictures. It really only comes down to ergonomics. For pretty much everyone on GFY (except for the half dozen high end pros) either camera would be a great choice.
Canon had the greater market share and always will. They are a much bigger company than Nikon. Nikon only makes cameras and lenses (and scopes, binoculars etc).
If I had to choose though personally just one camera it would be Nikon
