The syntax of this clause, which is ambiguous, is not the deciding factor. I would suggest that the exegesis move downward (not upward) from the paragraph and higher levels of discourse which serve as constraints on how the clause is understood. Keeping that in mind, we might ask: is there a comparison being made? If so what is being compared to what?
2 Peter 1: 16a
Ου γαρ σεσοφισμενοις μυθοις εξακολουθησαντες ...
For we did not follow cleverly devised myths ...
2 Peter 1: 19a
και εχομεν βεβαιοτερον τον προφητικον λογον ...
And we have something more sure, the prophetic word ESV
So we have the prophetic message more fully confirmed. NRSV
If we think a comparison is being made, it seems very improbable the Peter the Second would cast a shadow over the apostolic wittiness to The Transfiguration by saying that the OT prophecies are a more solid basis for faith. That would be shooting yourself in the foot, if you were a first century apologist for the Messianic credentials of Jesus. It is far more probable that the comparison, if there is one, is to σεσοφισμενοις μυθοις “cleverly devised myths.”
On the other hand, it is quite possible that no comparison is being made at all, Peter II is saying that The Transfiguration served to authenticate the prophetic wittiness. That is the reading I would go with.
C. Stirling Bartholomew