As you consider your response, keep in mind that you do not have to choose quality or service, despite the question’s structure. Use your response to discuss the merits of each. It is good to relate this question to your experience. Here’s a sample response below:

I believe that the two go hand in hand. You are not helping your customers by selling an inferior product. I make sure that the products that I represent are all of a high quality and good value, which gives me the confidence that leads to great service.