I don't know about the functional differences, but I have been using BIAB Windows version for several years and using this forum as well, and it seems that the Win version is updated much more often and is (as far as I can tell) first to incorporate new technologies such as Real Tracks and the other add-ons. I don't know that this will remain the case, but from a business standpoint I suspect PG Music will more often than not spend their resources on the format that produces the most customers and revenue, and that is the Windows platform. Not a statement of regarding one machine to be better than the other, just economics.
If you have a Mac and don't want to shell out for a PC platform, you can run either version, although I don't know how well that would work for you if you chose the Windows program.
Good luck.