Nikon D600: Are the "benefits" of this camera really worth it over the Canon 6D?

I have always used zoom lens (my latest being the 24-70 L F/2.8 II with the 5D3) and as early as my 40D and subsequent 7D periods when I used a 17-55 f/2.8 IS, I always experienced a shadow created by the built in flash whenever my zoom lens was extended a bit on the tele side. So, even if my camera would have a built in flash, I would not or, at most, use it minimally because of this shadow problem.