Are all presuppositions equally valid? If not, how does one determine which are more valid than others?

Internal consistency is the most common method I know of for determining the truth value of presuppositions. The test of internal consistency is what presuppositional reasoning is all about.

If, for example, a view presupposes "A" and "not-A" at the same time and in the same way, then, in that worldview, reasoning, knowledge, learning, meaning, morality, science, mathematics -- everything -- becomes utterly incoherent.

Alternatively, if a view presupposes "A" and "B", and "B" necessarily entails "not-A", then, yet again, that view is incoherent. Atheism is a perfect example of a view with this problem.

Now if universal incoherence is presupposed as part of the incoherent view, well, that's fine. But it never is.

So ultimately, by presupposing coherence and also presupposing contradictory concepts, the view that contradicts itself is self-defeating.

On the other hand, if a view does not presuppose contradictory concepts, and if that view does not presuppose concepts that necessarily entail contradictions, then it passes the internal consistency test. Christianity is a great example of a worldview that passes the internal consistency test.