I wouldn't take Highway 20 over the coast range. Very busy and no shoulder for a lot of it.
This spring, I got a chance to ride Logsden Road in from the coast, and it was fantastic. It starts in Siletz, which you can get to either by going north from Toledo or south from Lincoln City. We went along the Siletz River, from Lincoln City, and it's a great road.
Logsden Road itself goes through Logsden to Nashville, where it connects to 180, which goes through Summit to Blodgett. In Blodgett you can get on Highway 20, and the shoulder is good all the way to Philomath. The back roads are very quiet and beautiful. The only issue with this route is that there's about 2 miles of gravel road over a summit just before you get to Nashville, but it's doable with 23 mm tires if you're careful.
I haven't ridden 34. It looks fantastic, though, and I've heard good things about the Alsea Scenic Byway.