I don't think there is a right or wrong answer here. It is all in how you want to travel.
Taking the highline, or hwy 2, is kind of a roundabout way to get to great falls. It is a 2 lane highway the whole way and is probably the longest route. However, there are little towns about every 20 miles. With Glasgow, Malta and Havre being decent sized little towns. The rest of the towns are tiny tiny. There is surprisingly a lot of traffic on this road and it is sometimes hard to pass. You might end up with a train of cars behind you at times.
Hwy 200 is the straightest route, but man is it desolate. 2 lane the whole way with a whole lot of nothing in between. Hardly any traffic and not many towns bigger than a bar and a couple of houses. Between Glendive and Lewistown, there is literally nothing. This road rivals the loneliest road that is in Nevada.
If you take I94 down to Billings, then up to Roundup and Grass Range then over to Great Falls. This route would be 4 lane interstate to Billings with a few decent towns to Great Falls. Billings is a larger city and has everything.
All of eastern MT is basically plains and rolling hills. Expect some potentially high winds out there. I live on the western side of the state but have been over there 100's of times.
The cool thing about some of the smaller towns in MT is they all offer camping in town. They all seem to have a city park that either has free camping or a $10 camping fee. A lot of them are right on a river or stream too.
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