Good question.
The work started with the extension lib, and then moved to the SDK which is not specific to Domino. The move required some code refactoring, thus the package names and some features changed. Now, because all the enhancements go to the SDK, we are in the process of consuming the SDK from the extlib, and removing the original classes in the extlib. I don't have an ETA yet, but I hope that this can be done in the next few weeks. When done, the old classes from the extlib will be removed.
This leads to this answer: if you need some features that are specific to XPages (ex: the REST datasource), then you have to use the extlib, and then you have to adapt your code when the SDK will be merged into the extlib. Else, you should directly use the SDK.