When developing songs, is it more effective to start with lyrics or melody?

Before writing I always sing a melody that comes to my head in "gibberish". A language that doesn't exist. After finding the melody and structure of the song I can then create rhymes and syllables that match the gibberish vocals. Depending on which language I do the gibberish, I mostly find matching words in the first try that I wanna keep in that place. So I just need to write a story around it. It's like half freestyle.
To me creating a melody guide first makes it easier to write lyrics.