I have the Summilux-M 50mm f/1.4 ASPH and find it to be more or less "the perfect lens" for me. Yes, it can be somewhat clinical at times - but wider open I find it to provide truly outstanding isolation and lovely bokeh with all the sharpness I could ask for - especially stopped down.
If you want to avoid a clinical lens, the APO is the wrong direction! Leica set out to create, literally, a perfect 50mm with it. It will absolutely shine on the Monochrom especially. But if it's character you want, you might want to look at the previous generation of Summilux or Summicron. A little more "softness" (they're still sharp), a little more flare... Tons more character.
There's also a
huge price difference... As you're probably aware, the APO is among the most expensive of Leica's current lenses. Not sure if that's a consideration. But with the savings, you could go on a nice trip somewhere to shoot that older Summicron!
I like that all my lenses share the 46mm filter size too.
FYI, the Summilux is essentially an APO lens as well; it's just not marked/marketed that way.