IMO, SR20 is an easier run than US2 and it's our preferred route. That nay be because we live out at the west end of the spur that goes to the State Ferry terminal in Anacortes. State DOT were finishing up some road improvements on the west side when we came through last year, but we haven't been over SR-20 so far this year. It's typically our route for an early September trip. If you're headed west, through Winthrop, it's a steady climb to the top, but it's a well maintained road and wider than US2. My 32' V-10 Class A ran 35 - 40 mph all the way up without getting hot. Washington Pass is about 1000 ft higher than Stevens, but the grades are manageable and the views are worth the trip.
Something to consider, between Concrete and Burlington is to take the South Skagit Highway instead of staying on 20. Headed west on 20, take the road to the south about 3/4 mile west of Concrete, just before the grocery store on the south side (not the cross-road where the concrete storage tanks are). After the trestle over the river, turn immediately right onto the South Skagit Highway. Eventually, you end up at a junction with Highway 9. Head north on 9 into Sedro-Woolley and back onto SR-20 headed west. A minor diversion, but a very pretty drive along the south bank of the river. I used to drive transit part way down that route.
