The kind of development you decide to do depends largely on what you need to accomplish and what you are capable of doing, so it's really an individual decision. For example, you may be a C++ programmer who is interested in creating a compiled plug-in for Autodesk Softimage; or you may be a technical director who just needs to automate some repetitive tasks.
The Scripting Command API—provides access to the same commands that drive the Autodesk Softimage user interface. This is a great API to get started with, since most commands are logged to history, so you can find out how to script in Softimage interactively.
For more information on these APIs, see Understanding the Softimage SDK Architecture.
In addition, the dotXSI file format provides a text-based file conversion format which comes with its own API, the Crosswalk SDK. Together, these features allow you to import and export information between Softimage and another 3D animation package or a game engine.