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Nikon did not make an exact rival of the original, "classic" 5D. When the D700 arrived, it was clearly more technologically advanced than the 5D, more of a rival of the 5D Mark II. The images actually produced by the original 5D, however, were not necessarily surpassed by the D700!
The clear Nikon rival of the 7D is the D300s. They do not match each other, feature-for-feature, but the sum total of all features make these two rivals, logically. I carefully researched the differences and similarities of these two, when I bought my first new DSLR in late 2010. Cameralabs was one of my main sources! Please do see Gordon's excellent reviews!
If, by 5D, you meant the current-generation 5D Mark III, well, it is not so clear that the D800 is a rival. These are, really, two different cameras for two different purposes. The better one depends upon how, and what, one shoots. I have read compelling narratives written by photographers who own and use both 5D Mark III and Nikon D800 cameras, each for different types of photography. As I am Nikon-Canon brand-neutral, it is possible I will do the same, someday.