I have a 5D Mkii and looked at getting the 5D mkiii. After checking it out, I went for the 6d, fantastic in low light. I do a lot of low light, (gigs) and night time photography and I'm very happy with it . I use a 24 - 70mm F/2.8 L lens. Hope this helps.
I have the mkii as well, as far as I'm concerned it handles noise fantastically compared to the old technology, it would be worth spending the extra for the same sort of thing. if it didn't do it very good I would suggest the mkiii, I would upgrade myself if it wasn't so expensive.