I've ridden the coast and the Cascades through Oregon, but not Washington and California. I prefer the Cascades, but there is
much more climbing. If you want to avoid lots of climbing, you may want to stick to the coast. You'll still have climbing, but much less.
My preferred route through the Oregon Cascades looks something like this. Highlights of the route include Century Drive outside of Bend, Crater Lake, and Rocky Point (IMO).
The coast is very beautiful. It would be hard to narrow down the scenic highlights of the coastal route, but #1 for me would be the California redwoods. I also like the stretch of highway from Walport to Florence and the bypass by Otter Rock. The traffic on the Oregon coast is the heaviest in the northern section. It's not as busy south of Newport. The coast route will be cooler, but will have more traffic. The coast route will have more amenities - stores, restaurants, hotels, etc. If it's very hot inland, the coast tends to be socked in with fog. Conversely, if it's really hot inland, it's really hot inland
.
I found these pictures on a supported tour website:
Oregon Coast
Oregon Cascades with Aufderheide