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Your maximum aperature range for quality astrophotography using a SCT (which will probably have to be off axis guided at that focal range) is likely 9.25" - 11".
That is presuming that your site with a pier is very well shaded for wind (like an astrolab) or you live in a place where wind won't affect your tracking.
So your OTA (and hopefully well matched to it camera or CCD) choice will be affected by target preferences and typical seeing quality at your locality. For that size SCT you also have to add in focusers, filters, OAG, two cameras, focal reducers if required and sturdy mounting bars - like Losmandy D series. All this adds weight and limits the maximum sized OTA you can put on your mount.
As you are mentioning 10" - 12" SCTs - I infer your targets are small, faint nebulae (not planets nor open clusters). If you can specify what targets you wish to eventually image - so we know how dim they are and what your exposure times will be - gives us a better idea of how excellent tracking and focus control you will need - and therefore what maximum load you can put on your mount and hope it will all work out.
Your choices listed seem reasonable - but better information about what you wish to image and the maximum lenght exposure you hope to take, and whether its at full SCT focal length - or if you intend to use a focal reducer (and what your cameras and focusers will be) will likely greatly improve the advice folk here can give you. Otherwise we are shooting a bit blind offering advice.
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