After selling my C5, I have been staying with a 4" refractor but it's too long for indoor purpose. Yes, the quality is better but the usability is lower.
I sold that 4" refractor eventually, and now I'm trying to do some planet observation and imaging again with my Ranger (my only telescope at the moment), and I soon feel the limitation.
Now without a heavier mount but only my iOptron Cube and my Takahashi TG-SP II, I can only opt for something like a C5 again. It's just unique in its class. By going to a C6, you are way heavier, approaching to the weight of a C8! Thus, there's little reason to go for a C6 except for its price. By going a step backward to a 4" Maksutov, again, it could still be heavier than a C5!
Guess what, I might go back for a C5 eventually. Again, Borg 101ED is another choice, could be better too! Of course, I could go a step further by upgrading my Cube to a higher model, but that would be too complicated for me right now.