Python also has the benefit of being in Maya and Motionbuilder, even Blender.
With Python, you're training up on a language that can be used across packages, and one that many houses use for their internal pipeline and tools. It easily has the most momentum right now as a scripting solution in the 3D business.
JScript is easier to use with the XSI SDK, and as Kim said, that's the bulk of what you'll have to get familiar with when learning to script XSI. Both are fine as a language within XSI, but not everyone is comfortable picking up new languages all the time, so it depends on how you feel about learning JScript when the industry is adopting Python en masse.
- Chris
