Ideally, a bit more information would be helpful. What is the exact specification for the SUV tire, and why is your tire shop calling it an SUV tire?
There is nothing specifically rated as an SUV tire. But there is a Light Truck tire, designated by the prefix LT, which are used on most SUV's. Chances are the "SUV tire" is actually a LT205/70R15.
Virtually all cars and most minivans use a "passenger" tire, designated with the prefix "P", such as P205/70R15. I expect this is what your Sienna uses.
A Light Truck (LT) tire tends to have somewhat more load carrying capacity (when inflated properly) and is more durable under rough conditions, than a Passenger tire.
But LT tires do this with a thicker, stronger 'carcass" or tire body. This means they are a bit less flexible. They provide a slightly rougher ride, and tend to run a bit hotter at high speeds. They might have slightly higher rolling resistance and get slightly worse fuel economy.
You can use an LT tire on your minivan. But overall, I would go to another tire shop and get a tire that is rated EXACTLY the same as your existing tire, or that which is recommended as proper replacement equipment by Toyota. Chances are this is a passenger snow tire, P205/70R15