Yeah basically from what I know the dealer tires are OEM approved but I'm not sure about the "good prices" you're saying because either way it's still gonna cost higher. I think you're paying for their potantial accountability if the tires f up so if you think that's worth it then go.
Stick with your own tire guy as long as he match the right specs for your Siena. Dealer prices are always higher (unless it offers you a very good promo).
When it comes to dealer tires, you're usually guaranteed that they'll match the original equipment quality and fit perfectly, not to mention the added bonus of a warranty and professional installation. However, they can be a bit pricier than what you might find at your local tire shop. If your local guy is trustworthy and offers decent prices, you could end up saving some cash while still getting a solid product
