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For imaging...can't imagine the extra aperture would make a huge difference. Many (if not all) the larger SCTs (non-Edge) are reported to be shipping with FASTAR now so if you're still undecided you could always speak to the supplier. One thing to bear in mind is that the central obstruction is substantially smaller on the C8 than the C9.25 and C11, so you'd want to be extra careful minimise the aperture incursion by your CCD/DSLR with the smaller scope.
Visually...the C9.25 has about 40% more mirror area so (all things being equal) will of course give a brighter show. However, while the 9.25 has a legendary status, there's no magic to em and the Edge scopes give a flatter and coma-free field...of course aperture wins if you're going after faint fuzzies. Personally I prefer the flatter sharper wide field so went for the Edge. If all you care about is mostly in the centre of the field, the aperture wins again.
How useful the vents are depends on your climate, as ultimately they're passive. I fitted the Tempest fans to mine and I'm very happy with them, cools (or warms) the scope in next to no time.
I bought the Edge 8 on the AVX and it's effortless. I haven't tried any imaging with that setup yet though as I have a modded EQ6 also... which seems to want a good polar alignment for good Gotos where the AVX seems to do a better job of that for visual.
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