PC: More software, more users (so more support etc.), Macs are usually more expensive, most computers come with Windows (so if buying a Mac, you may not get the computer you want)
Mac: some say it is easier to use, less viruses
Alternatives
Linux: Free software/operating system, less viruses, more options in terms of opperating system
But... less commercial software than others (free software is often not as good as commercial ones, especially at high end), very few computers come with it installed, can be a bit difficult to get working
This is just for starters, there are lots of other reasons why, search it on Google.