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I have a C6 and recently bought a used Skywatcher Star Travel 102mm f/4.9 for the same reason as you. It is substantially brighter than an 80mm scope and has greater resolution. On deep sky it's very nice and is better than expected on the moon, Jupiter and Saturn. The only minor caution is that at f/4.9 it needs decent eyepieces to perform well at the edges. I have 11mm and 14mm Maxvision/Explore Scientific 82° eyepieces and they are pretty good.
The choice between a 90mm achro and 80mm ED is more difficult. The 90mm would only be 26% brighter. Ignoring the price I'd probably go for the superior optics of the 80mm.
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