both diet and exercise are important. but diet is the key. it is what causes either a cut or a bulk. some people can bulk on little to no food, some need mass quantities of calories. you need to learn your body.
think of it like this. food is the fire and fuel to keep you going..exercise is the quick flash of spark that ignites the fire.
depending on your body type..esp if you are endo or ecto, eating will feel more like a job than the lifting does.