There is a big difference between the Google Glass Mirror API and the GDK. The Mirror API lets you build web-based applications that interact with Glass users, while the GDK lets you build native applications that run directly on Glass.
The Mirror API is much easier to use than the GDK, and it offers a lot more flexibility in terms of what kinds of applications you can build. However, the GDK is necessary if you want to access all of the features of Glass, and it offers better performance than the Mirror API.