Nikon D600 vs. Canon 6D in Low Light?

Absolutely, flash is neither allowed nor needed for catwalk.

But I also go to TONS of parties. Carrying around 2kg a camera is bad enough. Adding a flashgun would not be ideal. As a fill flash (typically at a flash comp of -0.7 to -1.7), I'm happy with the pop-up on the D600. The only place it fails is the banner photos, given the harsh shadows it casts on the banner. But for photographing friends in the middle of the room / out on the roof deck, the pop-up does an OK job.