I've got to agree with Doorag. It really depends on two things: you and your trip. If you're traveling for the fun of the ride, then highways are probably the way to go. If you are traveling for the destination, the freeway gets you there. If you want a little of both, then plan a route that leaves you options. It isn't always possible, but if you can wake up and decide for yourself each morning of your trip if you are taking the scenic or the speedy path that day, you will enjoy it more.
That said, don't always assume the slab is the boring route! Anyone who has driven the interstate from Reno to Sacramento or Portland, OR to The Dalles, OR should agree.
All in all, plan the trip you want around the roads you want to take and the sights you want to see, and you cannot go far wrong.
What are you headed to Eugene for? As a fellow forum member and Eugene resident myself, I might be around to buy you a beer when you get here. When is the trip?