From my personal experience PC is more flexible and supports more games and programs. MAC is great for media programs and programs of that nature. For example we use PROTOOLS which is a music recording studio program and MAC rocks for that. PC studio programs like QBASE, etc, are not bad, but not as awesome as PROTOOLS.
PC also as mentioned already is upgradable, MAC is not.
My PC is already 6 years old but will still play any brand new game as I bought the top of line (at the time) Ahtlon 3.2 GHZ processor with 1 GB RAM and 256 MB 3D video card. If I was to upgrade the mother board, RAM, etc, it would be fairly reasonnable cost wise and then would have an up to date machine. So PC in a way is a better bang for the buck, but it does depend what you are doing with the computer. Games, e-mails, business and Internet, PC is the way to go. I have owned PC’s since they first came around and have only had a few virus issues, mostly with WIN 98 OS system. If you have a good anti-virus program you do not have issues. At my work we use Avast with XP OS and we have never been infected, so it actually works well. It can slow the PC down a little bit when first opening the Internet Explorer or Outlook program, but that is the only drawback. :-)