Use the writing implement or medium that works for you and your journal. If a Visconti Rembrandt fountain pen makes you eager to write, go for that investment. If a store brand pen which you can buy in packs of 50 gets your creative juices flowing (because you’re not worried about someone “borrowing” it), then use that inexpensive pen.
Sometimes you’re not even using a writing implement. There may be times when you glue an image to a page to speak for your mood instead of words.
The pen (or other medium) you should use for the journal is the one that serves you.