Too many unknowns to give advice, as Brian said. Why FX and why just these two models? What do you shoot and in what conditions? How do you display the images, screen, large format printer, home ink jet A4 printer, web publication etc? The D800 is a great camera but few people really need it or can use it for best advantage. Any of the modern DSLRs Nikon produces, even the the D3200 are capable of low noise, high resolution, accurate color, and metering and hard to tell the results between them except in extreme cases. Unless you have a compelling reason to invest the amount needed to optimize the lenses in your bag, a D7100 would be pretty hard to leave off your list. It is the best bang for the buck now in Nikon.
What specific problems with your current camera and lenses that some features of newer cameras would resolve? Is budget a prime consideration when selecting a new camera? Stan St Petersburg Russia