To me, the 17/1.8 from Ming Tien's review shows that, image sharpness wise compared to 20/1.7, they are more or less the same. CA, 17/1.8 is better. Even sharpness across the frame, 17/1.8 is better.
The price diff (if 17/1.8 is $799, compared to 20/1.7 at $520), of about $280 is for metal construction, better lens element and coating (less CA), and much faster focusing.
How much all these is worth the extra price paid is really up to the individual.
The 20/1.7 will always be a classic lens everybody wants.