First, I bought my D800 before the D600 existed. So there was really no choice.
Now that there is, the body style/form/quality would still push me to the D800. The D800 is a pro-style body with a larger form factor and more hard buttons. The D600 is a prosumer body style (think D90 vs D300). It's smaller in hand, has fewer hard buttons, and is less ruggedly built. Those differences alone push me to the D800. The extra resolution of the D800 suits me more than faster FPS. Also, the D800 high ISO performance is very very good, so I'm more than satisfied with my choice.
If you're looking to get into FX, have used prosumer bodies rather than pro bodies, and don't need the extra resolution, the D600 is a nice way to move up to FX.