How are they different technologically and in terms of practical usage (e.g. different types of datasets available, etc)?
Researched around but can't find a clear answer.
For a high-level comparison, B-PIPE is just a data stream and SAPI is a traditional API. Both B-PIPE and SAPI give access to the same data that powers Bloomberg Terminal but that is effectively all that B-PIPE does whereas SAPI provides a more diverse interface for building out more robust applications. If all you need is the Bloomberg Terminal data stream, then B-PIPE is the more economical choice. However, if you need additional features then B-PIPE will not do but SAPI may suffice, depending on what you need.
UPDATED LINK: Here is the documentation for a more in-depth look at the two technologies.