"Should I stay on I-90?" I think that depends on how big a hurry you are to get to Sioux Falls and how adventurous you are.
I am an Everett native but years ago I lived for a period of my life in Rapid City. I-90 is the best way to hog out the miles between here and S.D. I have never taken US-212 between Billings and Spearfish, but I have driven quite a few roads over the years which looked straight and flat between 2 points on a map, and I was often rewarded by unexpected scenery and highlights.
If you're in a hurry stay on I-90. If you want to drive through small towns and scenery that most of us will never experience take US-212.