The Advanced kit comes with Raspbian for Robots, a different environment than DexterOS.
Raspbian for Robots offers full freedom with the robot, if you are an advanced user (familiar with Linux, familiar with Python, etc)
DexterOS is easier to get started with. The same hardware platform lets you have both. If you reach the limits of DexterOS, you are able to burn Raspbian for Robots on the SD card and do more stuff (like controlling your robot from Twitter, if you wanted to)