But if we hypothesize that the difference between the two theories does not pay rent in anticipated experience, then I'm unconvinced that it is rational to say that one theory has higher probability.

If I offered two competing theories:

• Each electron contains inside it a tiny little angel that is happy when things go well in the world and sad when things go badly. But there's absolutely no way to detect such from the outside.
• Electrons don't actually contain any entities with minds inside them, even undetectable ones.

I think you'd assign higher probability to the latter theory, even though there's no difference in anticipated experience between the two of them.