Its not that simple.
Speakers provide realistic impact and weight. A soundstage in which instruments can be placed just as if they were in real life. It provides a reasonable amount of detail and accuracy of course depending on the cost of your speakers.
Headphones provide a sound that is much smaller, lacks weight, impact, and can not create a realistic image. However headphones can provide detail and accuracy that speakers of a similar price can not. Also for the obvious reasons such as headphones are portable.
Im going to go with speakers though. Headphones have always sounded to me like what they are....headphones, small cans over your ears. They cannot reproduce a realistic audio experience like speakers can