I'm doing the SF to LA AIDS ride on a Trek T-100 Tandem. It's a large because we both are. It's a hybrid tandem, but we changed the tires, wheels and cassette to have some good climbing gears. We may get new wheels before the ride to make it lighter.
I think I would classify ours as a "beginner" tandem, though I've rode others and am not sure what the difference would be.
The most important thing is to make sure it fits both the captain and the stoker.