The only advantage the 100/2.0 has over the 100/2.8L IS Macro is size. The macro is quite a bit longer...
For portaiture... we hardly ever actually shoot at f/2.0
Formally, something more like f/5.6 or a half stop more... to get eyes and ears both in focus - which also brings the nose into focus.
However, if you're really looking for selective focus... yes, shooting at f/2.0 or with the macro f/2.8 is up to you.
You'll never, notice the difference in DOF between 100/2.0 and 100/2.8. You'd have to sit there and A-B compare over again to see any appreciable difference. You'll like the 100/2.0 for its size, but really.... in dim light, I'd take the 100/2.8L with its IS any day. And thats exactly what I did!
IQ is beyond fantastically excellent on both lenses... if the ergonomics don't matter all that much... the 100/2.0 will stay home.