The Boise-Spokane is operated by Northwestern, usually a H3-45 or 102EL3. The Missoula-Minneapolis is operated by Jefferson, usually a J4500 or D4505. The Portland-Denver is operated by Greyhound, usually a D4505.
As for seats, Northwestern uses multiple models, but all quite comfortable. Jefferson uses the FAINSA Brasil Standard, which is OK in comfort, and Greyhound uses the Painful Premier in the D4505, which is, obviously, a painful seat. I'm talking seats here, not legroom. Now, the Greyhound is fine if you don't get a D4505. For example, you could get a Denver- or Los Angeles-based X3-45.
Anything else you need to know, feel free to ask, but I will probably geek out on you.