To me, CoH's main appeal compared to CO is that CoH seems to treat the subject matter more seriously. I always felt that the characters and lore in CO leaned too far towards campiness.
To use an analogy, CO is like the 1960s TV "Batman", while CoH is more like Tim Burton's first "Batman" movie (with a little "Dark Knight Trilogy" mixed in). The former is good for some laughs, but the action, situations, and dialogue can be pretty atrocious. Whereas, the latter has better characters and dialogue, and a better sense of realism.