Can't compare the two, since i have only used the FA 24 mm before. I contemplated selling this off and getting the voigtlander 20mm, but after going out and taking shots with it, I am well satisfied with mine - even if I do get the 20mm, I think I'd still keep the FA*.
I'm no professional, so I'm not really that particular about the edge to edge sharpness of the lens. I have used the lens for different types of photography, but the one that really stands out in its capability to function in low light, wide open, with quite decent images (to me at least). I've no complaints about it in all the shots I've taken with it.
Lastly, here are just a variety of different images taken with the lens, for you to judge yourself:
landscape:
bokehs:
backlight
low light