Using the mouse is always better for games because it is more accurate, and when playing WoW you definitely need the mouse for camera movement and stuff, using a trackpad for that is too jumpy and not fun. There are very few games that a trackpad works well with. Also the magic mouse is not very good for gaming either in my opinion because it’s not as fast in terms of buttons. What happens when you want to press both left and right buttons at the same time? Or middle click? Again, it only works well for specific types of gaming.
However, I have both and for me, it is the trackpad for web browsing and emails and light work. However, I am faster with the Magic Mouse in applications where I am doing work or need better accuracy.