I had fun with CO but eventually grew bored with it when it first came out. I'd say it's the sadder of the two, primarily because it could have had so much more development. Instead it became a cess-pool of scrubs and "Stop having fun!" guys who had zero tolerance for how others played the game. Many players acted like sado-masochists wanting CO to get some gruelingly difficult content when the game had non, so the devs ended up adding "Punishing" rather than "challenging" content to the game. Long iteration times, trial/error, poorly thought out puzzles, team play which forces the holy trinity and no other alternative, killed the game. Not to mention balance passes designed to cater to the scrubs and shfg's.
And course, a lack of things like buffs/debuffs/cc, and attacks which all do nothing but damage with effects which are mostly just there for "moar damage" with debuff effects that don't even stack proper.
DCUO? Well I can say it has had a leg up on gameplay being distinct, which I did like. I didn't like that it went towards generic raid content and more generic raid content right away after hitting level 30 and a repetitive solo path for gearing up the skinner box :/. DCUO didnt get boring combat wise, but I feel it appealed to much to those who only cared about gear, which I feel is a prominent problem in mmorpg's; to many players who only just want better and better(ad infinitum) gear and power levels.
I liked CoH not just cause I could make a legitimately powerful character, not just because of it's ever increasingly rich details n lore, but also that it had high replayability. I mean energy blast knockbacks, dark blast to-hit debuffs(in every attack no less), radiation blast defense debuffs or even simple fire blast or say sonic blasts -resistances, and the capability of stacking them with other effects or players of the same set, it created room to always try a new character once in a while for a fresher experience. CoH never got old for me cause of that replayability while CO and DCUO both just get old for me.