I prefer 94 over 90 given similar weather conditions but both will get you there. The stretch from sturgis, to montana can suck big time in the snow on I-90. The drive is 22 ish hours from Milwaukee with dry roads. Add in weather, and it could be 2 days, which is just part of the deal driving west in the winter. We have been stopped by closed roads about 25% of the time when traveling in January/February.
The drive either route is pretty laid back when the roads are dry. Add in some snow and wind, and it will be a pants shizzing, white knuckle experience where you feel like you are about to crash at any moment for hours on end. All part of the experience I guess!