Is the D800 better value than the D600?

50% more MP, because of inverse square law, is 25% more sensor resolution. Image resolution is part derived from lens resolution and part from sensor resolution. While the lens plays a part, on average at optimum aperture, about 15% more resolution is the likely result.

I am not convinced the viewfinder is "bigger". Both are 100% and 0.7x so about the same. That said the D600 screen is about half a stop brighter, but less suitable for judging depth of field. If you mean AF point screen coverage (something different) there is a distinct D800 advantage.

USB 3 can be much faster - on a USB 3 computer

The other +/- issues may or may not be important depending on your style of photography.

You did not mention D800 has slightly better f8 AF capability.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.