Across the Rockies
 

If flying from Montana to Aurora OR, what route through the mountains would you take? And then if flying from San Diego to Phoenix, what route through the mountains would you take?

San Diego to Phoenix
 

I fly this route often. The MOA's and restricted airspace make direct flight nearly impossible. Also, the terrain south of I-10 is rough and inhospitable should you have engine trouble or need to land. So, what I do is fly NE over the north edge of the Salton Sea and join I-10 past the palm springs TRSA, then follow the highway in to PHX. This works for me. ymmv
Ron

For Montana to Aurora

I'd follow I-90 through the Rockies then follow the Columbia River Gorge through the Cascades.

For your South route:

JLI V460 SHADI V64 BLH T306 BXK

For the North:

As previously stated but with a stop at 38S for cheap gas.

Montana's a big state, so it depends, but unless you are way far north or south in Montana to start, I'd say take Mullan pass from Montana to Aurora.

In reality there are a lot of good route options in that area, but that pass is a non-issue in most any airplane (as long as the weather's okay) and is quite straight-forward. If it wasn't labelled as a pass, you probably wouldn't even recognize it from the air. Relatively benign, terrain-wise, and the highway is there. There's a weather reporting station located on the pass, as well.