Actually, Softimage XSI is no longer developed or available, you can't purchase it as far as I know, although there maybe downloads still available. It was a nice program that I enjoyed using a lot, but without current development it may not be worth the many hours it takes to learn, depending on what you want to do with it.

Blender is a free program that might be of interest to you if you are getting started. It has a rather high learning curve though but it is easier than it used to be and is still maintained and developed - last I checked.

Depending on what you want to do there are many more 3D graphics programs out there and they come in a wide range of pricing. I would be careful before purchasing expensive packages.