We have a Sienna that I love. It's roomy, rides like a car, and almost paid off. No maintenance problems at all (typical for a Toyota). We thought the Sienna rode a little more like a car than the Highlander and the Sienna was a bit bigger. The Highlander we test drove had a bench seat for the second row. Our Sienna has captain's chairs in the middle row, bench seat (seats 3) that fold down in the back. Both the second and third row in the Sienna are removable.
Highlander is an SUV, while the Sienna is a mini-van.