Over the Cascades in Washington US-2 is a 2 lane highway for the most part. I-90 is a 6+lane freeway and 4 lanes east of the mts. US-2 is very scenic and the grades on both highways are manageable. You need to be on your game driving US-2 on the west side and through the Cascades because this road because it IS a 2 lane highway...lots of lethal accidents between the small towns on the west side. In my opinion US-2 east of the mts. is more scenic than I-90 because it passes through apple country in the Wenatchee Valley and then farm country small towns between Wenatchee and Spokane.
If you have the time and are comfortable with the conditions I describe I'd take US-2 all the way from Fall City to Glacier NP. Drive WA-203 north from Fall City to Monroe where you pick up US-2