Oregon it is. Washington doesn't have that much accessible coastline and activities.
It would be very hard with miles of slow traveling to tour the whole Oregon coast from one spot. I would suggest you pick two for a couple weeks each..one toward the north and another south. By doing this you could also venture into Washington a short distance and also into California and the redwood country. There are also some nice areas inland to check out from the coast.
Your month will go by fast. Have a good trip!