I've only played the demo of From Dust but from what I can judge I would recommend Bastion by far. I really enjoyed playing Bastion, the gameplay is fun, everything about the presentation is stunning and it implements some really great origional ideas.
From Dust seems like a fun game but I got a bit frustrated with the demo and it seems like the novelty wears off pretty fast. To conclude, they both seem worth getting but Bastion is a game I wouldn't recommend anyone pass up.