Both, really. There are elements of art and creativity to it, of course – knowing how to put a creative campaign together; being able to write compelling copy; knowing what design and text elements will cut through the clutter; finding unique market opportunities for a product or service.
But there is also a great amount of science to it – analysis that goes beyond a subjective opinion. The Latin word scientia has many meanings, mainly knowledge, science, skill. Science can be defined as the knowledge of something acquired by study.
Much of marketing is based on science – knowledge acquired by study. When we run a marketing campaign, we study the results. We compare results with a previous state, we analyse responses, we use benchmarks to identify changes. Market research is based on statistics and quantitative analysis. Marketing plans are driven by return on investment (ROI) and making sure that efforts help drive revenue and, in most cases, profit. Reading the marketplace involves looking at sales strategies, consumer trends, and the economy.
So a good marketing director is someone who has solid business experience and an ability to understand the numbers. Save the artistic work for the creative directors and designers.