i would like to point some things out, champions is about to open its beta and dc is about to go into alpha as its already in it pre-alpha phase. I mean you do know that dc has been shown and tried out by many fans at multiple press events, releasing hands-on videos on the internet as well as leaked ones from development. They already have the base system for the game, however in the alpha/beta they'll probably make small changes.
Personelly i liked their original idea where they had different servers and on each server a player would be a dc hero like superman or a dc villain like luther with all their powers and of course being able to advance. Their new idea of custom characters with dc heroes doesn't really appeal to me, probably what makes me want to go play champions.
Being the green lantern would have been awesome, but if i'm going to be a small hero instead of one of the greats i'll go with champions so i can customize him to the max. In champions you'll probably be able to have any power and more from dc, same for the costumes. I mean you really just gotta compare the two character creators. Plus in dc you have to choose a powerset, but in champions you can choose powers with absolutely no restriction, enabling your character to have multiple powers. Im gonna shoot you with electricity, light, gamma radiation, energy, fire, ice, and rocks.
If your a marvel guy like me, i would definetly go with champions as it has more of a marvely feel to it. You could always tell the difference between dc and marvel by their costume. Dc has batman, superman, flash, basically just tights with colors. Where as marvel has the x-men with his their awesome jackets, the hulk with his purple shorts, and galactus with his mechanized suit of ultimate armor.