20mm on OMD concerns

I have the 17mm f1.8 also, it's noticeably wider in focal length, as well as physical dimensions.  Not quite as sharp, but decent.  Gets some funky looking lens flares too, which may not be quite to everyone's tastes.  But it focuses fast, quiet, has a lot less purple fringing, and no more banding.  I shoot with it and don't have to think as much about limitations.

But for portability, I guess you'll have to go for one of the pancakes, the 17mm f1.8 sticks out too much to really make any of the camera's pocket-able.