Contrary to popular belief - neither coffee or tea wake you up or give you energy, in fact if you actually look at the calorie content of either it's virtually nothing. Your body likes to keep energy in reserve for things such as flight-or-fight, so - in order to do that - it releases adenosine which convinces you don't have much energy so as your body can maintain reserves for instances where you actually need it.

Coffee contains caffeine which is structurally similar to adenosine. When consumed, it sneaks into these receptors and blocks them, preventing adenosine from binding - hence, you're awareness of your actual energy reserves is no longer blocked. Caffeine also signals your body to produce adrenaline, which is why you might feel jittery or get that "boosted" sensation even though you haven't actually consumed any calories. 

Tea's the same, it's just milder and contains L-theanine, which promotes relaxation without causing drowsiness by increasing alpha brain waves, which are associated with alert relaxation.

When combined with caffeine, it smooths out the experience, giving you the focus without the "jitters" or anxiety often caused by coffee - hence why some people prefer tea over coffee, including taste.