Bonjour Ulisses,
XSI has the same workflow philosophy as SI3D. Except that it is much more superior: You can creates buttons, toolbars, customize layouts. Also, scripting capabilities extend its functionality.
On the other hand, I think that XSI is not as stable as SI3D. I experience more crashes, sometimes the software just stop working (while taking 100% of cpus), and there some instabilities with rendering (altough I suspect that there are do to system configuration and hardware). Of course XSI 2 is more stable than 1.5.
The other negative thing about XSI, imho, is the lack of certain features, like complete nurbs tools and rigid bodies, among other things. But I think that eventually the software will have all what is needed.
I can't compare with Lightwave, sorry.
You should order the demo. There are a lot of videos, all the documentation, and of course a free copy of the software. Obviously you can't save with this free copy, and scripting capabilities are disabled.
Salutations - Cheers
Bernard Lebel