Don't get me started on EQ (GEM) mounts! I admit I don't like them, for all the usual reasons that visual astronomers who don't like them don't like them: they put the observer in uncomfortable positions, the movements are unnatural for human beings, they're heavier than comparable alt-az mounts, they require polar alignment. My first telescope came with a GEM, and I've had several telescopes since then that came with GEMs. But over the years, I've gotten rid of all of the GEMs. I like alt-az mounts.
That said, I'm not really 100% satisfied with any mount I've ever owned, including the ones I own now. There is always something - or several things - that doesn't quite work ideally. They are all compromises in one way or another.
But the saving grace of a C5 is that it is light and compact, so it will work better on mounts that heavier and/or longer telescopes won't work so well on.
One type of alt-az mount I'd stay away from for a C5 is one that has the telescope positioned directly on top of the mount, like a photo tripod, even one with a fluid head. The C5 is just heavy enough that when it is pointed toward zenith, it will tend to fall backwards on these mounts. I have a 501HD fluid-head. The C5 is borderline - actually goes over the border - for that mount.
Get an alt-az mount that attaches the scope at the side.
An eq mount can be fine....
....as long as it's set up in alt az mode