Don't know whether you've already made your purchase, but here's some additional input. Based on my personnal experience, I would go with the Toyota product. I did get an 03 Ody brand new and have never been satisfied with my purchase. All sorts of little things break on that car: power locks, lock cylinder (you can't get the key in anymore), multiple trim elements just come undone... all pretty annoying. Perhaps they've improved the vehicle over the years but I won't buy them again.
On the other hand, I've owned a 96 Toyota (Corolla) which I got new and never had any problems with it - truly only reg maintenance. It's got close to 200 000kms. If this is indicative of Toyota's product quality across the board, I'd think that the Sienna would be a better purchase.