How do you get a lens, that is too good for portraiture? "Back in the day", you smeared gel of some kind on the edges of your very best lens and it became a nice portrait lens. Today, it is easy to soften with filters or in PP. You are going to PP any portrait anyway, aren't you? Then the lens is good for any number of uses. I want the best lens I can afford.
My daughter is visiting from out of state, and has the Oly 45mm with her. I tried it briefly, and found it too soft for my liking. But I don't do portraiture very often. For that use, it would be a wonderful lens, in my opinion. And to back up my thoughts on the subject; she was going to give it to me, as she is traveling all the time and doesn't have time for portraiture. I told her, thank you, but just put it back, for the time, you will need it. I really like the lens, but it is not exactly what I want. Not critically sharp enough for my liking.
Would I buy the Sigma with 75mm? Sure. If I had that much extra money laying around.