I've been playing Champions Online on and off since it launched and more solidly the past few years. I absolutely prefer CO's costume creator and think it's the best ever on an MMO. But, now that I've tried CoH again with Homecoming, I can confirm - for me - that is the only advantage I see with Champions.
Gameplay for CO just feels clunky to me? The targeting doesn't work as well. Having to worry about your energy builder. And powers just don't feel as powerful, somehow.
The community has some pockets of good people (I am something of a quasi-member of one group of former CoH players), but overall CO is a very cliq-ish community and chat has plenty of toxicity. And things like costume contests often feel like they are actually about popularity.
The gear and amount of stats, it just feels needlessly convoluted. Ultimately, you're gonna look up a build and follow a cookie cutter if you want to compete - so why not just make it simpler, to begin with. CoH's enhancement system is so much more sensible and well-designed to me.
Paragon City feels more like a real city. And it also feels like there is more to do. For me, the radio/newspaper missions, in particular - I just love randomly generated content, it keeps me hooked in a game like this, gives me a consistent well of things to go to if I choose to stream, for instance. Closest CO has to that are things like Alerts and UNTIL missions and both of those seem way more repetitive to me.
I love the way my hero looks on Champions Online (he's pretty much my ideal MMO character). But if I could lift him out of Champions Online and put him in City of Heroes, I'd never look at CO again. Heh. (I might not even play another MMO, were that the case)