I have the Panny 14/2.5 and the AF is acceptable (but slower than Oly lens) in daylight, but at night it totally cannot focus without assist lamp, even under bright orchard road light.
My 45/1.8 tho hunts a little at night, is able to lock focus.
The fastest focus of all in low light condition is actually the 12-50 kit lens
If the 17/1.8 can focus like the kit lens i will be very happy. Any faster will be fantastic.
Image quality wise, the Panny 14/2.5 is sharp in the middle, but has quite a bit of vignetting and not as sharp at the edge. It would have been a good portrait lens, but this is 28mm equivalent field of view... so, it makes this lens not so appropriate.
I guess the 20/1.7 is the same (since it shares about the same lens construction with the 14/2.5, with 3 aspherical elements). Sharp in the middle, less vignetting (due to the fact of the focal length) but not as sharp at the edge.
Therefore, i still choose Oly 17/1.8 because its sharpness is more or less up there, but much faster AF. Plus, i really prefer Olympus colour from its lens and also prefer 35(4)mm focal length. Ordered mine at HN, collecting it later.