Enlarging an image in postprocessing rarely gives completely satisfactory results. It really depends on what your final use for the image is. If a 9 MP image will be sufficient, then you can crop the shot from the 5DIII. if you need more megapixels for cropping or printing large, then that might not be the best way to go. The 7D is, in effect, an optically perfect 1.6x teleconverter.
Personally, I am getting a 1D X. It will replace my 5DII, but I'm keeping my 7D for birds and wildlife (with the 100-400mm).