Though I don't know if this is still true but when I bought maya 6 it came with both a mac and a pc version as well as a Linix and IRIX version as well. Other then the OS it is running on the program is the same on mac and pc. So only advantages are how it will fit in with whatever pipeline you already have set up.
Be aware that Maya has (Don't know about Maya 8) has never ran as well on a MAC as it does on a PC.
Search around, there are lots of threads about this, mostly due to video card limitations but also there are small quirks on the MAC version not found on the PC. This might have all been cleaned up in Maya 8 but was the case for Maya 7 and earlier.
Yeah, it's always been that way, yet the final authorization you recieve from alias will only work for the OS you intially request. In other words you can't later on down the road switch your license to a different OS (This might be chaninging/already changed). I know people that have migrated there license from 1 OS to another but requires a lot of work with Alias/Autodesk.