I have a strong 3dsmax bias, only because i’ve used it longer than Maya or XSI. But they are ALL great packages. I’ve seen too much great work created in all 3 programs to be able to say one is ‘better’ than the other. To answer you question in a very general way, i believe Maya has a stronger grip on the film industry, with 3dsmax having a stronger grip on the video games industry. This is in NO WAY a hard rule. There are exceptions all over the place. A good place for you to start would probably be the discreet, alias, and softimage websites. Their show reels should give you a good idea about what kind of work the pros are producing with each package.
But like Darkone says, a lot of the major players write their own software, which would mean if you were hired there, you’d have to be trained anyway. What i would recommend before upgrading, download the max trial version, maya and XSI both have free learning editions. Play. Explore. If one package feels more comfortable then that means something. If you develop strong skills as a modeler, texture artist, animator, etc, and you understand the CONCEPTS, not just which button does what, then it won’t matter which package you’ve worked on, you’ll be able to switch without much difficulty.
Best of luck with your decision,