I have both the Voigtlander Nokton 58mm f1.4 and FA50mm f1.7. It is day and night. The VL58mm is way superior, much better IQ, more pleasing colour rendering. No competition.
I did a short, limited comparison between the VL58mm and FA31mm:
FA31mm f1.8 vs VL Nokton 58mm f1.4 - PentaxForums.com
The VL58mm is very close to the FA31mm IMHO.
I have used my VL58mm f1.4 for now 8 years. It was my 2nd lens, and I bought it based upon advice from this forum. This was my best lens investment. The VL58mm is very sturdy and perfect for harsh outdoor conditions. Especially the VL58mm is extra-ordinary (IMHO) in poor lighting conditions: e.g., fog, smoke, smog. In such poor conditions, the VL58mm is my best lens (ahead of FA31mm and FA77mm). Lastly the VL58mm is excellent in very-very-low light when the AF does not lock. The MF ring of the VL58mm is very smooth and easy to use.
I advise you be be patient and get a VL58mm with PK mount. There are some appearing in the marketplace here and on the bay relatively regularly. The VL58mm is worth the wait; you will enjoy it.
Hope that the comment may help.