Definitely Mac for me. I used to only use Windows (for around 8 years) but then got a Mac and now I cringe when I have to use my Dell.
To those who say PCs have more compatible software, I tell you this: Mac has more compatible software. I can run all Mac, Windows, and Linux programs on my MacBook Pro (I play Steam games) - can you do that? I believe that makes Mac much more compatible than a PC, no?
Now for gaming, I'm a heavy gamer. I routinely play Team Fortress 2, Portal, Half-Life 2, and Unreal Tournament on my Mac. All run smoothly without any lag whatsoever.
To the person above (prox741) me who said that Microsoft has market dominance and is therefore better: obviously it has market dominance, it makes cheap products. China has market dominance over almost everything, I guess their products must be better than anything made here in the United States, right? I guess a Toyota is better than a Rolls Royce too.
Seriously, most of these topics are completely pointless. The majority of the people who respond to them have never extensively used more than one operating system and are therefore poor judges of the other system's capabilities.