Personally I lean towards accepting antiquity, but there are lots of interesting problems, and I am basically undecided. The private papers of Smith containing puzzled letters he wrote to colleagues about the contents of Secret Mark point to sincerity, unless he was being really, really clever, trying to create a nuanced fraudulent background for authenticity by writing letters he never made public. That really doesn’t make any sense.
The other question is, if Secret Mark is ancient, who wrote it in antiquity?
Anyway, in all things I follow the two fundamental principles of the historiographical method of Herodotus (simultaneously the Father of History and the Father of Lies):
1- When there are two conflicting stories, the most interesting one is true.
2- Never let the truth get in the way of a good story.