it depend, manga u can kind of customize the experience and read as fast or as slow as you want, with anime its easier to just rot and watch, with manga its better when im invested in the universe since i can get way more out of reading than anime, but with anime you get the visual experience better and its easier to pick up body lang through voices than just words
if the source material is originally from a manga and its not an anime original id say you can watch it first, then if you like it a lot just read it, thats what i do