I made the switch from Maya to Softimage. The first time I tried to learn Softimage I was a bit confused and went back to Maya, but then almost a year later I gave it a second go and things started to click.
For the most part, the two programs do exactly the same thing. There really isn’t much that one package can do that the other one cannot. And if your using Mental Ray, then your shaders and render settings are nearly identical.
The big difference really comes down to the layout and interface as well as the fluidity of the tools that are built into the package out of the box.
For a major studio who hires full time programmers to customize and re-build their software and can afford to buy any plug-in on the market, then Maya may offer some distinct advantages and it will be a great asset for you to know and understand Maya’s interface in case you get a job at one of these places.
However out of the box Softimage is entirely ready to go for a smaller studios and individual artists, with a lot of thought put into how things are organized and used by modelers, animators, etc… And for me, while the tools and ideas in both packages are very similar, it’s Softimages streamlined and well organized approach that really sets it apart. Where as with Maya, I always found myself customizing hot keys and shelves trying to get the feel of my workflow comfortable and productive, but with XSI, almost all of the commands and tools I am looking for, are readily available by default.
Some of my favorite things about XSI are it’s default hotkey/interface, the Compound Shaders, ICE, and the powerful but simple to use FX tree.
P.S. When you first launch Softimage you get an interaction model option that will actually allow you to use Maya defaults, however if you choose not to use the Maya navigation option, and instead take advantage of the default Softimage interface, it may take slightly longer to learn but you will get more out of the program in the long run. If you choose this route I highly recommend immediately going to File>Preferences>Tools>Camera and turning OFF “wheel zoom” as this will allow you to use the mouse wheel to adjust your brush size when painting weights or using the proportional modeling brush. I also recommend changing File>Preferences>Interaction then click on the “Tools” tab, and check ON both Enable Sticky boxes, one of which is now turned off by default. Both of these suggestions actually used to be the defaults in XSI prior to the release of 7.0