Great question. Seth Godin said it best; Pick one.
It is both, however, I think marketers get into trouble when they believe they're conducting one or the other, marketing or science - AT THE SAME TIME, like it's some sort of magical gift or craft that allows them to weave seamlessly in and out of both. The trick to good Marketing is to know (& respect) when art is being performed and when science is being performed, and to treat each as such. Wear the ART hat when your artists are busy crafting & creating.. and wear the science hat when you're computing spreadsheets and ROI. Simple. I think the craft of Marketing is very much like crafting a good story or storytelling in general. When storytelling - we weave independent considerations together, like DATA, and AUDIENCE & a mythical story. Three independent considerations that are masterfully woven together as one compelling story.
The trick, once again, is to NOT try to do it all at the same time. Take the time to consider each as independent of one another....this is the 'art' of storytelling - or shall I say the 'science' of storytelling.