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At that focal length and optical design 100% go for OAG or ONAG; differential flexure is a killer so you absolutely want to keep it rigid and simple. Add a really good mono imaging camera for your guiding and PHD2 is a good starting point!
Side by side may work at times and in a pinch - but if you want deep, long exposures at that focal length - and have to fight the SCT's mirror varying with elevation shift, flop, tilt or other factors (which might only be thousandths of an inch) then you definitely want to guide on the same optical path that you image from!
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