Having ridden through some of the Columbia Rver Gorge and the section of the Northern Tier east to Glacier N.P. twice, I personally would pick the latter, although I cannot comment on suitability of MT 200 to the junction with the Northern Tier since I took the Great Parks North Route between Whitefish/Columbia Falls to Missoula. Just keep in mind that picking up the Northern Tier that way (you would join the route at the junction of MT 200 and MT 56, and it leaves MT 200 for a while heading towards Clark Fork, ID.) will ultimately send you over four mountain passes in a row in WA, culminating with an epic and long downhill from Rainy Pass on the North Cascades Highway.
Another option is to simply stay on the Trans Am to the coast. I did much of the OR section of the route during Cycle Oregon 2002. While I fond some of eastern and central OR somewhat borning (and hot and dry even in early September), McKenzie Pass east of Sisters is a terrific ride. It's an old volcano that blew a long time ago. At the pass, there is nothing but black lava rock. There is an observatory built out of the stuff. It looks like a place Skeletor would call home. The climb up the east side is not that bad. The west side has a cool section of tight switchbacks through forest. Descend with caution.