I must admit a major factor was that it is simply much more sexy than the MacBook. But in reality I was moving up from a 12" PowerBook (also Aluminium) and didn't want to move to plastic. Back then, 12" Alu G4 versus cheap plastic G3 iBook for nearly the same price was a no brainer, but it has stuck this time and I haven't minded spending nearly twice as much on the pro model!
I love the backlit keyboard, the extra power, the GPU, the bigger screen size, multi-touch, etc. The MacBook is great value but not half as sexy looking and I'm not so keen on the keyboard.
Up to you though. To be honest, both are powerful enough for most tasks, but choose the MBP for games. Go check them both out in the store and see which one you like best.