I have mostly given up trying to keep my digital journal in anything other than org-journal. I do also use Day One as a photo log, and sometimes I consider moving my daily journals back there, but it doesn't happen.
In order to hedge my bets, I'm sending each day's org-journal entry to Day One using the following bit of elisp:
(defun jab-dayone-add-note () "Sends contents of buffer as Day One entry" (interactive) (shell-command-on-region (point-min) (point-max) "/usr/local/bin/dayone2 -j=Journal new" nil))
It's not fancy. The entire buffer is sent as a new entry to Day One, using the current date. At some point I may get fancy and use the org-journal entry's date and possibly include tags. For now, this is almost like cross-posting journal entries from Org to Day One, so I don't have to decide which to use.