Illuminations was superior to Harmonious, and that's despite the fact they allowed aspects of the show to fall in to disrepair. And that doesn't even take into account what Harmonious does to the lagoon daytime views.
That said, HS has no fireworks show, so deciding which to skip between HS and Epcot is pretty easy. Once Fantasmic is back, I'd likely skip Harmonious if making a choice, though it could depend on the changes being made to Fantasmic.