Downtown Disney:
This area is much larger than CityWalk. You have many more places to choose from if you want to eat. DD features many quick service and sit down restaurants. There are also more than 35 places to shop. But, it is not within walking distance of any of the theme parks, so it is not as convenient. If you are here, you might as well stay for a while as opposed to a quick stop-by.
CityWalk:
Located right next to both theme parks, CW is easily accessible anytime of the day. This area also has many restaurants, and places to shop. CW is advertised more towards adults because it has many bars and nightclubs.
Dining
Shopping
It is hard to decide which theme park "extra area" is better. DD is much larger and has more options for shopping, eating, and entertainment, but, CW is more convenient located and is perfect for a quick visit to shop or dine. Both areas compliment their theme parks nicely.
:)