Bonjour JTA,
This is a very though question you're asking. I will try to be as objective as possible, but you'll see that many people will argue on what I say. In fact, this is an endless debate that's been raging for years.
1-If Maya is widely used, is it a strong software then?
Yes it is a strong software, otherwise it wouldn't be used in production by big companies. But the main reason why Maya is so widely used is not only because of its strenghts, but because of the way it is marketed. Consider Alias Wavefront as the 3D Microsoft: poor product, but kick ass marketing. For example, altough I have no proof of that, many people said that Alias is offering important discounts to big studios, and in return these studios can't say that they used other softwares than Maya...
I say poor because there is more and more people that come to XSI and say that they moved from Maya because of many problems. It is rumored that AW is facing serious development issues regarding Maya, some say that the implementation of new functions becomes harder wich each new version, because of bad design.
The truth, if there is, is that Maya was an extremely powerful product when it was at version 2 and 3. But its architecture became cumbercome. AW releases new version at a surprising fast rate, but it looks like this speed comes with a price: quality.
2-If XSI is powerful, then how come it isnt widely used?
There are several reasons for that. First, XSI is still considered a young product, it still has to prove itself (production-wise). Many people will tell you that XSI is production-ready, and I agree with them. But there are still some aspects that refrain XSI:
-XSI still lacks some features, like strong nurbs modeling and Renderman intregration. You know that big studios use Renderman most of the time, and maybe they can develop a XSI-Renderman pipeline. But the more common user will not.
-Implementing a new software in a productin pipeline can pose problems. I suppose that many companies want to be absolutely sure that XSI is reliable before using it full time.
-Also, many companies develop in-house scripts and tools intended for the software. So to implement a new software means that you have to rewrite many things. Consequence: there may be some production delays, and not everybody can afford that.
-Finally, and sadly, Softimage does not put the same effort in its marketing than Alias.
If these 2 softwares are completely different for what they are used for, can anyone tell me what each software is good for?
I will not develop on that, you just have to dig through this forum to see people arguing that each option is better in one software than on the other. From what I know, Maya is strong with modeling, sub-division modeling, particles, soft and hard bodies, cloth, fur, and scripting. XSI is strong with almost everything, but its greatest strenghts, imho, are the workflow and the rendering. Rendering is a huge weakness in Maya.
Bon! So, if anyone think that I'm dead wrong well I don't care, this was an opinion based on my knowledge and experience. I don't want to start the typical software war and I welcome anyone who thinks differently.
Hope this answer your questions!
Salutations - Cheers
Bernard Lebel