Louis, I own both of them.
The 17/2.8, after a few days, I attached a Ricoh 0.75X converter to it and it has not come off yet.
The 20/1.7, I am still not comfortable with that FL for street. I do use it for IR (street) just because it focuses faster in IR and also is a better lens in IR.
In short, both the lenses as such are not that great.
One advantage (a great advantage) of the 17/2.8 is its close focus distance. ~10cm.
I did post on this earlier (there is a thread somewhere), I guess no one has figured it out yet.
DISCLAIMER (YOUR OWN RISK!)
Put the cam on AF mode and pull the front of the lens (preferably with a hood attached to grip it), the focus barrel will extend further than it does and will get stuck at ~10cm close focus. To reset, turn the camera off and back on to resume normal functions.
The Pana 20/1.7 did this to a lesser extent in the beginning but after doing a F/W update, it will not do it now.