The short answer, both. You can develop a great product, but without good design it will fail. Conversely, you can have a great design, but without a good back-end/front-end architecture, no one will be able to use the product. Good advice, and often thrown to the side by many, Putorti and Patzer suggest you have the engineers and designers sit in close proximity to each other and talk, thus facilitating collaboration. They say,  “You don’t throw stuff at engineers and tell them to build something. They aren’t code monkeys.” Basically engineers plus designers equal a great team.