I would say the writer. It's the writer who puts the story together and needs to have continuity, knows the characters and their personality, etc. Peter David's run on Hulk is one example. Stan Lee's scripts on the Fantastic Four and Spider-man runs are legendary. Even so, the artist is also vital. Without the proper strips colored and drawn properly, even a kick-ass script can suffer from lackluster art. Ditko was a great artist who could make the most mundane science fiction comic book strip (from the 1950s anyway) seem interesting and exciting. Same with Frank Miller and Gene Colan, both extraordinary artists. So I guess you're right. There is no wrong answer. They're both vital.