They are both PC's. Mac is just a brand of computer made by Apple. I run on a Macbook, but use both OS X and Windows 7.
Sometimes you just need to use Windows (video games, essentially). As it has been stated, many developers may not make Mac versions of their software. So, depending on what you are doing, this should be taken into account.
I use OS X a bit more heavily than W7 still, because of use in the terminal and the loads of useful system utilities. It is a UNIX based operating system after all.
Speaking of Unix, this question is essentially entirely moot anyway. The big difference between Macs and other name brand PC's is that only PC's with that little apple on them can run OS X (legally). All PC's, including Macs, can run Windows, Linux (Ubuntu is a great distribution), other UNIX based systems, etc.