Derek Sivers types his journal entries in plaintext files. I use paper. Recently, I've been wondering if his way's better...
As neither plaintext nor eyes will be going out of style anytime soon, both formats should be readable forever. But text files are searchable and replicable. They have the added bonus that you can encrypt them to keep your thoughts private. Also, as Derek says, you could theoretically load your all your life's digital journal writing into an (offline) LLM, turning it into the ultimate tool for self-understanding and advice.
A paper-based journal can do none of that, so why do I still use one? Because neither medium is better than the other; they're just better for different things.
If you want to organise your thoughts, type your journal. Plaintext's ability to be edited, refactored and organised makes writing in it far more conclusive. A digital journal entry could start as jumbled mind vomit but end up as a structured thesis on what you're thinking through - with headings and everything! It's a fantastic tool for gaining actionable insights from a swirling mind.
Where plaintext journals are for organisation, paper journals are for exploration. How can you ever hope to organise your swirling mind if you don't know what's in there to begin with? Their lack of structure, inablility to be edited without visible scars being left, and the fact no files need to be named makes paper the perfect place to dump your most raw, disorganised thoughts.
Words are printed on paper, not merely written. You can't go back and hide what you needed to say at the time without it being obvious. Paper is the perfect place for stream-of-consciousness journalling, prying out thoughts from the subconscious mind. I find that being in direct contact with the words I'm writing (I am making those symbols - not a computer; they're in my handwriting, not someone else's typeface) makes me write most earnestly.
So the answer: should you journal on paper or in digital files? I do both. It just depends on whether I want to explore my thoughts, or organise them.