Yeah I've never seen an implementation of OTR that blocked archival—in the journalistic sense, it's still on the record so to speak. (That being said, OTR's deniable authentication means I can use my plaintext archive to recall the conversation but I can't prove to a third party that it's from who I say it was—nor would I be able to if I was storing the encrypted conversation.)

With the above considered, I'm curious to hear another argument for OpenPGP over XMPP rather than OTR.

(Edit: Should say that there likely are OTR implementations that archive the ciphertext, or don't automatically archive at all. However, having used OTR in Jitsi and Adium, both I believe keep regular old plaintext logs.)