Hi Rachel,
I see no one has managed to address this yet.
The answer, I suspect lies in what you want. The 50D has a smaller sensor, so you'll have a narrower field of view for any given lens. The 5D a larger sensor (avoiding the potentially inflamatory 'full frame') so if you have any ef-s lenses they won't fit or work on a 5D. A new 50D will have a warranty, higher frame rates and may have better af than a secondhand 5D.
In terms of 'image quality', there isn't really enough information out there yet, but I didn't feel that the 50D sample images put online by Canon showed an advance over the 5D (I could be wrong though).
Mike