DC Universe Online = Champions Online = City of Hero/Villains
I've played each of them extensively, especially CoH/V. They're all almost exactly the same game. However, CoH/V and DCUO require a monthly fee.
If you're looking for an MMO until TOR, check out Champs Online first simply because it's F2P. Get a toon to 20 and decide if you like it or not. If you DO like it, consider DCUO or CoH/V trials to see which ever-so-slightly-different game you might prefer.
Literally, they're almost the same game.
You are painting with a pretty broad brush there. Many colors are found in each....
They all do include Superheroes, I guess.
DCUO has twitch based combat with a limited set of powers, costumes, one useable power bar, and some questionable writing. Limited amount of content. It's an MMO for the PS3, designed for use with a controller. Poor chat interface. Many reports of empty servers.
Champions Online has probably the best character creator around, but the store is pretty terribly implemented. Their drop loot system of adding to powers is convoluted and a bit messy. More content than DCUO, but still not a huge amount. Free play doesn't allow you to choose your power sets now, you just get a limited number of templates.
CoH/V has the advantage of years of continually developed content. The disadvantage that parts of the game were built before WoW. Character creator is second only to CO. It has user created content. It's more of an alt driven game, but they are adding end game content.
DCUO has such limited content, that the OP would probably be through it in a couple of weeks. Might not hold interest til TOR hits. CO is better at that, but still fairly limited. If you are interested in stories, CoH/V has lots more to it, though the missions can be repetitive, especially if you are working through the older designed content. But there is a bunch more of it.