The difference seems pretty minuscule. That said, # 2 makes slightly more conceptual sense, but really my vote would go for # 3: if you've got a congregation that can sing Gregorian, and you're not doing something like interpolating polyphonic choir parts into it, then just have the congregation sing the whole thing.

I didn't know there was an option besides having the congregation sing all of it.

This practice is something common here. I live in France and I have seen recently option # 1 in a EF mass celebrated by a priest from the FSSP (so it is likely not to be completely untraditional) and option # 2 in a OF gregorian mass. Reading your comments (“I didn't know there was an option besides having the congregation sing all of it”, “I've not witnessed this over here”), I realize this is not common in the United States.

I have noticed that in masses at the Vatican the custom seems to be to sing the parts of the plainchant ordinary in alternation between schola and congregation (with the organ lumbering in when the congregation are supposed to sing).

What M.J. Osborn describes, with the organ lumbering, is exactly what I am used to in gregorian masses*, here on the other side of the Atlantic. But there is not always a consensus about who is singing what. That was the point of my question.

* I never heard an alternation between choir and congregation for a Sanctus in a non-gregorian mass. The Gloria, though, is always sung alterned, whether it is a gregorian one or a modern/french one.