Which is better? A Nikon D600 or D800? Mostly using it HANDHELD & with low light.

Confidently get the right tool for the job.

Obviously... (and arguably if we're talking D600/D7000 owner perspective) the D800 is a professional grade camera similar to the D700 whereas the D600 is a consumer/enthusiast camera similar to the D7000.  Unless you're 9 years old, I'm sure that you can muster the discipline needed for the higher resolution, and if not, you also have the ability to set to hit the QUAL button and turn the command dial to NORM.

If some elite photographer tells you you're not good enough to shoot with a D800, shame on them.