You'll be climbing big mountains no matter which way you go.
#2 is no picnic in a U-Haul. On I-90 between Bozeman and Butte there are some pretty big passes to climb. From Missoula to just inside Idaho there's a fair amount of climbing as well. Lookout pass, which is on the ID/MT border is the biggest. Then in Idaho you have 2 passes to Coeur D'alene that are a little less severe but still a lot of elevation change. From CDA you're done with mountains for about 250 miles until you hit Snoqualmie.
I'd say #2 is the most scenic as well, but I'm biased since I live on that route
You might encounter some traffic in Spokane depending on the time of day. But even at its worst Spokane traffic isn't too bad, relative to Denver, LA, Seattle, etc.