I am a scientists
The winner is sadly not clear-cut, and depends on the situation, as coffee and tea are stronger in various circumstances.

For example, when it's raining outside and you have a digestive biscuit, tea wins in a 3.2 second knockout.

When sitting on a pavement cafe in italy watching girls on scooters going "ciao", espresso takes the lead.

And when working really late to finish an important job, crappy nescafe made with not-quite-hot-enough water and weird milk substitute stuff is THE FUCKING TITS.
( Mockingbird Practitioner of SCIENCE(TM), Tue 8 Apr 2008, 11:48, archived)

Please bear in mind though
that tea is the only legal option in the U.K.
( wheelybird Paul's out of toilet paper - send donations, Tue 8 Apr 2008, 12:00, archived)

of course, I'm not encouraging kids to experiment with illegal caffeinated beverages.
Tea kids: you know it makes sense.
( Mockingbird Practitioner of SCIENCE(TM), Tue 8 Apr 2008, 12:05, archived)