Data?
Many people reach for a cup of coffee when they need to concentrate. People with flagging focus might, however, get more bang for their buck with a cup of tea.
A 2007 study conducted by Doctor Foxe found that theanine and caffeine synergise to boost the activity of brain neurons.
With coffee I just feel energized but that energy isn't directed. With good tea (the loose leaf kind) I feel both energized and focused. I could never explain it but the article above might be the reason.