Nowadays, these companies are essentially interchangeable. There is a standard way that comics are written, and that crossovers are handled, and how they are rebooted, etc. Writers go from one company to the other like it's no big deal. But pre-Marvel, DC was not very good. They could be fun, but disposable entertainment. There was no characterization to speak of, which reflects the fact that they were writing comics for very young boys. Stan Lee, Jack Kirby, and Steve Ditko then came along, and changed the game fundamentally. DC eventually had to respond, but it took a good, long while (really the late '60s) before they could even hope to compete with what Marvel was doing on a quality writing level. So yes, DC developed the Atom, and Green Arrow, and the Flash first. But Ant-Man, Hawkeye, and Quicksilver were much more interesting for a long while.
