I've used two routes: one through Pendleton, OR and Logan, UT, the other over I-90. Both work well.
On the first route, I was traveling from Everett, WA to North Carolina. There were loads of thunderstorms in MT over the mtns and on the east side of the mtns, which made the I-90 route a bad choice. So I flew from Everett south to the Columbia River, then to Pendleton, OR, then to Logan, UT (day 1). On Day 2, crossing the Wasatch Range I had the option of climbing over the mtns at Logan (which I did, going up to 10,000' to get a big bunch of terrain clearance), or following I-80 from SLC to Evanston, WY. Once across the Wasatch there were no more mountains to cross, and I had great views of a number of mountain ranges in Colorado to the south. The visibility was tremendous, the surface winds were pretty strong (often 20-25 kts), and there were fun things to see along the way, like a beautiful Stearman flying westbound between Rawlings and Evanston, and the 737 doing touch and goes in the pattern at Cheyenne. Once east of Cheyenne the humidity went up, and the visibility went down, but there were still plenty of good people to meet and interesting things to see.
I did the I-90 route last summer, and it was a snap. Again I was traveling West to East. Crossing the Cascades on the Bellingham to Spokane leg I used Stevens Pass (you can also use Snoqualamie Pass if you are approaching from a more southerly direction). On the Spokane - Helena leg I was flying below the mountain tops, in the early morning, and it was smooth sailing the whole way. However, visibility was rather poor due to the multitudes of forest fires. There were a couple of places approaching the east side of the mountains where I had to deviate away from the interstate due to TFRs for the firebombers. Once east of the mountains it was easy flying, although not particularly scenic and there were plenty of thunderstorms to avoid. The good thing is that visibility was wonderful, and it was easy to see the thunderstorms from far off. I was amazed by the size of the ranches - those homes are really far apart - and decided ranch life would be well suited to introverts. My route took me through Helena, MT (where the Red Hot Skillet Lickers were playing) and over the Custer Battlefield (very interesting to see from the air, and definitely worth adding to your itinerary if time permits). South Dakota was hot as the devil, and a "cold" MOA I was transiting went "hot" when an F-16 had a mechanical problem and needed an unplanned inflight refueling. St. Louis approach control was decidedly unhelpful and refused to give me flight following, despite the plethora of thunderstorms and commercial traffic I was working my way through on the east side of the city.
Have a great time. Whichever route you chose, you're sure to meet some great people and see some unexpected things.
Eric