I live in Spokane and have driven the 1 route 7 - 9 times each over the years - and the 2 route 15 or so times.
The roads are good except for the occasional construction zone. their are two main routes:
1. I90 to Mt93 at Wye (near Missoula). This is freeway to the cutoff and 75 mph 2 lane up to Flathead lake. I would recommend the east side to go around Flathead Lake to Kalispell for the view.
2. The North route through Coeur D' Alene, Bonner's Ferry, Libby and Troy is a nice 75 MPH 2 lane route.
The North route is 1/2 to 1 hour longer but the more picturesque route, Troy is a neat little town with lots of way cool Eagle sculptures.
Don't miss Kootenai Falls about midway between Troy and Libby - but it is a 3 mile hike with some up and down to get to the falls from the parking lot - not for the faint of heart.
I usually take the North Route due to less traffic - but that's just me.