Of course, this whole debate about the 'suitability' of one atmosphere or another for life is presuming that there are distinct 'types' of atmospheres. This is even more UNrealistic than trying to claim that life could exist in a predominantly chlorine atmosphere.
I hope that SE V will have much subtler gradations of 'suitability' to account for the highly variable composition of various elements that would exist in real atmospheres. Balancing atmospheric gases could be a field of terraforming all to itself. (As opposed to the very crude 'conVersion' currently offered.)
Then with temperature, gravity, and radiation levels we could finally have some believably realistic environmental management on our worlds.