John1701a has said that he is planning an upgrade of his Prius page on the subject of tires for the '04 Prius. (Currently he has recommendations for the Classic.) His page is at http://john1701a.com
It is strongly recommended that whatever tires you use, you use them all around.
I used to use studded snow tires on my Civic. They're legal in ND only during the winter months. I didn't have separate rims.
Go to the tire store where you would have your winter tires switched (assuming you decide not to use all-season tires all year long) and ask them three questions:
1. What would an extra set of rims cost?
2. What's the cost to switch a set of tires including mounting and balancing?
3. What's the cost to switch tires if they're already mounted?
Then ask yourself how long you'll have the car and do the math. Then decide whether the up-front cost of rims is worth the yearly savings.
Personally, I drive so few miles that I'm going to stick with all-season tires. I understand that modern all-season tires are nearly as good as studded ones, and with all the electronic stuff in the Prius (trac, VSC, ABS) I don't feel I need studs. (Note: VSC is not in all packages.)