MET, but I am a giant fan of GB and will own a pro at some point in the future.
MET has one of the best rulessets in pinball, period. Even if you hate the music/theme, you have to admire the game design.
The feature stacking in the game is insane and can lead to monster games. Very intense with a few good players. One good multiball with stacked modes carrying in can turn a game around in a hurry and is always a close game.
Working through CIUs is fun, the dots are amazing (esp with a colorDMD).
GB pro is great but needs a while for the code to come together to make it compete. MET is done and done and plays beautifully. Get one now, trade for a GB in the future possibly. Always demand for a MET, even if there are a lot of them out there