Leon,
Just to elaborate on what Tony said, the major difference is field of view with the 5D.
As an example a 4" f5 scope would yield a field of view of about 2.5 degrees wide with the 350D. With the 5D it would be closer to 4 degrees. Furthermore the larger pixels in the 5D mean that it is more sensitive to extended objects like nebula.
Taking this further to get the same 2.5 degree field of view with the 5D you can now employ a larger 6" f5 scope. So for the imaging the same area of sky the 5D will have over 2x the sensitivity of the 350D. However you will need highly corrected optics to realise this gain.
Terry
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