The first thing that popped into my mind: equal complexity branches would have equal probability. We know that's totally not how it works: take quantum bits, for example. You can have arbitrary superpositions of both states, and it's very hard to argue that one suddenly acquires a higher complexity than the other.
I haven't fully thought this through, but there is probably some way to include the state transition itself in a way that makes that work. I don't think I know enough physics to figure out whether the concept's generally salvageable.
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