Depends on what you're looking for. Laptops are a lot more portable. Even the most extreme laptop can still be put in a backpack and transported around ( even if it does weigh 20lbs. ) Also, if you're worried about price, you'll get a lot more performance out of a $500 desktop than you will out of a $500 laptop.
So, what are your priorities? What kind of games do you want to play, and at what resolution? Do you need an OS? Need new peripherals like a monitor or keyboard? What is your budget maximum?