Age old debate. I am gonna go with coffee. It's just more tasty. Tea is boring.
I also have a prediction...the fans of traditional prog rock/metal (DT, Queensryche, Spock's, Rush, etc) or just plain hard rock will lean towards coffee, and the fans of more experimental, alternative stuff (Porcupine Tree, Opeth, Radiohead, etc) will lean towards tea. Let's see how my prediction plays out.