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There is actually a very good article on cloudy nights which explains in a very easy to follow manner the relative merits and recommendations of a prisim v mirror diagonal. Naturally, I can't seem to find it at the moment - but I am sure if you hunt around you will locate it.
As a very simple rule of thumb (and there are exceptions/caveats to this point) mirror diagonals are generally better for short focal ratio scopes (f/9 and less) and prisims are generally better for long focal ratios.
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