A grip for your D700 would be a lot cheaper. The IQ of the D700 is the same as the D3.
I have a gripped D700 which uses the EN-EL4a batteries and shoots 8 frames per second. The only reason I also have a D3s is to shoot bands in dark bars where the extra high ISO capability allows me to get shots I could not get with the D700. Naturally, since I have it I use it all the time. My typical landscape kit is the D700 and D3s each with a different lens. I have no trouble going from the D700 to the D3s and back. I also have a D2x and a D2h and a D300. I find the FX cameras to be far superior.
You might consider having a kit of one D700 with grip and one D700 without.