You absolutely need both. I don't even think both the product and the marketing have to be the second coming level good or anything either, they just need to be there. It's hard enough to get both things right though.A good product or skill alone is not enough, because without traffic and the ability to reach the right audience, it's very difficult to produce tangible results. Through experimentation in many different fields, I've realized that promoting and bringing the product to customers is just as important as building a quality product.