My experience would say to stick to the interstate as much as possible.
Come in on I-84 - Main issue will be coming down the hill out east of Pendelton.
When you come into Portland - Take I 205 on the east side and it take you south and tie in to I-5 just south of Portland.
Just do not enter POrtland during rush hour .
Then take I-5 south and take main road into Grants Pass.
I would not Take 20 and other roads through Bend - Most are 2 lane roads going over a pass and down through canyons.