Depends on what you want.
The FA31ltd is unique.
Its a 31mm focal length so it is a bit wider and sits between a 28mm and 35mm in terms of working distance and FOV coverage.
So its an even more environmental portrait lens compared to a 35mm, but not needing to get as close to the subject as a 28mm.
For its f1.8 and wider FOV, its also a small lens (and small lenses go places).
Its mainly more center sharp wide open, but if you don't shoot wall charts, this emphasis on central sharpness results in a good pop to the images.
The 'pop' is really good between f2-f4 (where the lens sharpens up really well over f1.8).
20160530-IMGP1469-BW by
jenkwang, on Flickr
20160527-IMGP0794 by
jenkwang, on Flickr
At f8 its very sharp edge to edge and coupled with that wider 31mm fov it works well as a 28mm sort of focal length for landscapes.
IMGP2951-PS by
jenkwang, on Flickr
Metal build, manual aperture ring and somewhat damped for manual focusing to top off the small perks of the lens.
The Sigma is a more modern lens.
Much bigger and designed to be sharper on the off center areas right from wide open.