The D700 has some advantages when it comes to low light, where you can use 2 or 3 ISO stops more than the D300, better colour rendition range, and fast focusing.
For motorsports where you can get close, e.g. MX or car rally, the FX D700 is the obvious choice. For other motorsports or long shots you will need larger lenses, not always the best choice for a long day's shooting where you may have to move a lot to get the best position. The DX D300 with its effective 1.5 magnification scores there.
However, the real deciding factor is the quality of the lens. Fast focus, VR, and availability of wide aperture, are what make good shots more often than sheer magnification. Also, lenses do not go out of fashion nor get superceded as often as the camera body. Lenses are more important and are the better investment in my opinion.
Ed