So you are coming from the south on 99 or I5, and want to go to the coast. And debating whether to take OR138 NW to OR38 and then west, or continue north to OR38 and take that the whole way (through Drain)?
I haven't driven either, but have looked at them with GM and streetview. I don't see much difference in width (ie. shoulders) and curves. I don't see any tunnels on OR138, but there is a significant one on OR38, at Elk Creek Tunnel Forest State Park (.2m with a slight curve). Signs warn of road narrowing and possible bikes in tunnel.