Some people think it doesn't really matter. I think it sorta does. I think the expensive Canon lenses look better than the third party lenses. Some think the third party lenses look just as good. To me an expensive Canon lens just looks better. I don't know why but on video I can just see it.

That being said I'm going to use third party lenses for video because I can't justify the cost of an L series lens right now.

28mm-Canon or Sigma
35mm-Canon or Zeiss
50mm-Canon, Sigma, or Zeiss
85mm-Canon or Zeiss

I think if you can get some sort of adaptor mount you can put Nikon glass on the 5D but I'm not sure. If you can Nikon will fit all of these lenses as well.