Using one or the other can depend on how long a dish is cooking. Dried herbs take a while to impart flavor so they're mostly useful for longer cooking dishes. Also, getting a piece of dried herb that hasn't had time to hydrate & soften can be really offputting to eat! Often, however, I use both- dried or sturdier herbs at the beginning & fresher/more tender herbs at the end when the dish is pulled off the heat. That way you get layers of flavor, not just top notes.