The short answer is, Performer was designed for vis-sim, while
  Inventor was designed to be more general purpose.  

  IRIS Performer is for developers who need to extract maximum
  performance from SGI machines for visual simulation, virtual
  reality, game development, and high-end CAD systems.  Often these
  applications need multi-processor Onyx systems with multiple
  RealityEngine pipelines with a high degree of parallelism and
  running at fixed frame rates.

  Inventor is designed for maximum programmer productivity when
  writing other kinds of 3D applications, like modelling, animation,
  visualization, etc.

  Both toolkits are general purpose enough that they could be extended
  into the domain of the other, but the question you should consider is
  "what is the *fundamental* goal of my graphics development?" If it's
  portability to non-SGI systems, easy X-window system integration, or
  handy graphic widgets, IRIS Inventor is for you.  If it's brochure-
  level performance in advanced graphic applications for the specific
  domains listed above, then IRIS Performer would be the likely tool.