Nikon D600 vs. Canon 6D in Low Light?

DPReview007 wrote:

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.