By comparison, the ride through the North Cascades, northern Idaho, and Glacier National Park is the crown jewel of the Northern Tier, in my experience. If I had limited time, I would enjoy the mountain scenery and hop on a train as needed to cross eastern MT, ND, and MN.
Have done Route 20 through the North Cascades to Glacier N.P. twice (and the entire Northern Tier once) and cannot agree more. Can't imagine trading it for interstate riding or U.S. 2 riding. The one place I would get off 20 is between Sedro-Woolley and Concrete. The S. Skagit Highway on the other side of the river is a gem. And you may not have much traffic on 20 up to Rainy & Washington Passes depending on when you go. Both times I did the climb in late May. The rode was virtually empty. Of course it snowed, but nothing heavy, and it didn’t stick.
Beyond WA…I would strongly advise avoiding U.S. 2 from Columbia Falls, MT to Glacier. There is a curvy section with no shoulders and some trucks. The route AC uses between these two points is nice. Was there last year. The unpaved portion was manageable on 37c tires. The ride up Going to the Sun to Logan Pass in Glacier is well worth any extra days.http://www.flickr.com/photos/davez2007/sets/72157620763740044/
The final 26 photos were taken on the west side of Logan Pass. There are ways to skip the portion into Canada is you are pressed for time.
The alternative, U.S. 2 from W. Glacier up to Marias Pass, is nothing spectacular, and it's a long, long slog up with narrow shoulders in most places.
How much time total do you anticipate having to get from Seattle to Minneapolis?