I have reams full of characters. It's one of the first things that I try when I get a new system. It's one of the first areas of concentration in my designs. That said, the characters cannot stand alone and a world that is suited to the characters, and the characters are suited to, is key in bringing any narrative alive. So as I create the characters, they breathe life into my concepts for the worlds.
I tend to build characters more than worlds, but the worlds I spend more time upon. All that said, I really can't choose between the two.