Well there's a question I thought I would never have to answer...
Sienna:
3.3L V6, about 200hp, 4000lb.
Century:
3.1L V6, about 180hp, 3500lb.
Both transmissions are super tall for highway cruises, and the Century should offer far better low-end torque.
I would say the Century. Now from a roll... I would go with the Sienna.
