Adventure Time is the all-around better show

Adventure time is probably better because it has a full 4 seasons of great material already.

Gravity Falls is only 12 episodes long so far. It's fantastic, and for time reasons it's probably the better choice for what to watch first.

Either way you can't go wrong. They're both really good cartoons.

I would actually watch Gravity Falls first, since there are only 12 episodes out and no information on when the next one is being released. Then, while you're waiting for more Gravity Falls, you can watch Adventure Time. It might keep you from chomping at the bit for new GF episodes.