Hey there,
So, both the superhero MMO's have free trials, so all you who haven't tried them can see what all the fuss we're causing in this forum is all about.
Champions Online has a free trial only for this weekend, so try it while you can. It's graphics intensive so compare your system to the requirements first.
www.champions-online.com
City of Heroes has a 14 day trial available at any time, found on the main page.
www.cityofheroes.com
Which game would be better for you? I play them both and here's a simple comparison.
If the costumes and the look of the character is your main consideration, try City of Heroes. They have an extensive costume creator and more pre-generated costume sets than Champions online. They also have special clothing and accessory packs including some very sexy clothes in the Wedding Pack and extra cyborg parts for all you bionics fans in the Cyborg Pack. Champions Online does have an extensive costume creation system as well, but doesn't have nearly the same amount of plain clothing, and the characters look a bit more cartoonish, IMO.
If the powers and abilities of the character are more important, try Champions Online, if your system can run it. Champions Online lets you do more superwoman things like picking up cars and buses and either throwing them or using them as a melee weapon. The way the powers are set up gives you much more control over your character's development than City of Heroes has as well. With Champions Online, you can create your super-strong, indestructible girl with laser vision, telekinetic push (super-breath), and any other power that makes sense to you, while in City of Heroes, you can have a Tanker character that is super-tough, but not super-strong and doesn't deal out tons of melee damage, that's the job of the Scrapper marital artist or swordswoman.
Anyways, hope this helps folks that may be on the fence and hopefully we can get a bigger population on these games going.