Your questions are kind of broad, but I'll try and answer some of them.
You shouldn't have any problem going from Maya Complete to Maya Unlimited on any platform. Mac or PC. As long as it's the same version, or newer. As far as working on a Mac or PC, Maya Complete is virtually the same on both platforms, so you won't have a problem there. Maya on the Mac has a few differences, mostly bonuses actually, like being able to read and render ALL quicktime compatible files, like .ps letting you use your native layered Photoshop files for textures. Most MEL scripts work, except for ones that require certain file paths for Windows, but if you're MEL savvy, you shouldn't have too much of a problem with it. As far as using what everyone else is using, PC's, that's entirely up to you. But as far as Maya goes, Mac or PC, it's the same, so whichever you get you will know both, because it's the same.
There are a ton of free plugins and such, for users who use Maya on the Mac. Jason Schleifer, works at Weta... you know, one of the guys who animated Gollum, and all around nice guy, uses a Mac and I don't see him complaining about it.
Check out his work while you're at it. He's working on a short that looks pretty interesting.
http://homepage.mac.com/jschleifer/HomePage/index.html