From carefully looking at the photos, and watching the video, it appears that this was either a "J" or "E" model MCI, based on the curved stairwell up front. Interesting, because Arrow Stage Lines has a lot of Setras in their fleet, altho this bus was from another Arrow division.
It is tragic that nine folk lost their lives in this, it is also amazing that so many survived with only minor injuries.
Seatbelts may or may not have helped in this situation, that's a debate that could rage for years - unsolved.
Goes without saying that weather was a contributing factor - black ice, anyone? Driver survived, so his input will be important to investigators as well as his HOS logs. Coach probably has a Series 60, so it's black box will also be studied, as well as any on-board video recording devices, if equipped. Same goes for any tachograph or similar instrumentation. All will take time. . .
Speculation as to whether an older GMC, Crown or Gillig might have withstood this better is simply that, speculation. The kinetic energy involved in a bus crash is HUGE, compared to an automobile. Building a coach to survive a roll-over intact at highway speeds would result in a vehicle similar to a military tank, with gallons per mile fuel economy. Not going to happen.
However, I would not be surprised to see the USDOT or NHTSA propose new structural integrity requirements out of this. It's pretty obvious, looking at all the glass in a new coach, compared to the older models, that the roof is not supported as well as before - at least to the layman.
Regardless of all this, our main thoughts and prayers should be for the families involved in this tragedy.
FWIW & HTH. . .