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@Ludd
Planets are such tiny targets that you really don't need a very big sensor. I currently image planets using a tiny 4.83mm x 3.63mm sensor, with only 1.3MP. The important trick is to take a huge number of frames in a very short space of time, and then stack them to improve the signal to noise ratio. On bright planets (i.e. Jupiter) I'm able to shoot at close to 100fps.
To get a decent-sized image on the chip you'll need a really long focal length; I usually shoot at about 5000mm (f25) which is perhaps at the long end of the range that gives the right relationship between the pixel size and the resolving power (aperture) of the scope. With that focal length, you have the challenge of getting the image onto the chip (that's one area where a big sensor might help) and keeping it there (that's why you can easily spend more on the mount than on the optics).
The 5D would be excellent for wide-field shots - big pixels mean high sensitivity and low noise - but if you can shoot video with it, you may get good results with planets too.