How about counterfactual predictive power and verification? If I could observe the inside of that box, then I could see a difference between the two theories.

I realize this opens a potential can of worms, i.e., what sort of counterfactuals are we allowed to consider? But in any case, this is how I've understood the basic idea of falsifiability. Compare to Yvain's logs of the universe idea. (He's doing something different with it, I know)