I can't really speak for Roll20, but I can tell you a little more about The City of Brass.
First a key difference, we're not a virtual tabletop. Our aim is really to enhance the in-person experience although The City of Brass will work wonderfully with groups that play over the web w/out the need of miniatures (or with another solution for a virtual tabletop).
For players, the system offers a convenient place to enter all your characters, regardless of the game you play. It's also very flexible, so you can input house rules and use them to influence elements about your character (such as ability scores, defenses, attacks, etc.). Your character sheet is also largely interactive, most elements have inbuilt dice rollers, for instance.
The Campaign Manager system will also be used by players to keep updated with where things are at in an ongoing game. It's intended to allow game masters to quickly post things such as game recaps and house rules. You can also get a look at other players character sheets (if they've been set to public) and the game master can view the private parts of the sheet also.
World Builder is a place to put together your setting. You can do all sorts of things in World Builder including interrelating elements of your setting automatically (things like Cities to Continents to Nations to Rulers to Villains, etc.). It also produces a very nice, clean output for others.
You can see an example of a setting created in World Builder here - valleyofthekingsrpg.com
The last major system is Story Builder where you write adventures, interconnect them with your creatures (that enjoy all the power and flexibility of characters).
If your interested, there are videos showcasing each of these systems (about 10 min. each) on the Kickstarter page.