This is actually a fairly simple decision to make.
Do you want to shoot at f/2 and above most of the time? Choose the FA 35mm f/2.
Do you want to shoot at f/2 and below most of the time?
Choose the 30mm f/1.4.
Test sigma 30_Da35
Here is a comparison between the Sigma 30mm and the DA 35mm f/2.8. The article is in French, but the pictures are self explanatory. The Sigma is kind of an interesting lens. Even at f/1.4 it's as sharp as the venerable FA31 at f/1.8, and is even sharper than the LTD stopped down. The border performance, however, is terrible and never really improves regardless of the f-stop.
The more you stop down the Sigma, the more you seem to expose the weaknesses that would otherwise be hidden by the rather good bokeh. Other than the poor edge resolution figures, the Sigma produces some weird geometry distortion in the corners.
I actually ran across this lens while browsing pictures on PhotoHito trying to find motivation to pull the trigger on a FA31 I was looking to buy. Here are a couple random shots:
IMGP8572 - ???????:photohito ??????? - ???????:photohito ??? - ???????:photohito ?????? - ???????:photohito
However, the FA35 is better in nearly every major category. It's smaller, lighter, has comparable center sharpness, far superior corner sharpness, supports full-frame, has less barrel distortion, less vignetting, less chromatic aberrations, and less focusing problems.
Nevertheless, simply by nature of its design specs(30mm f/1.4), the Sigma provides something the FA35 cannot. The Sigma is closer to being a true normal on a APS frame, and the large max aperture gives it a very film/full frame DoF look. So for low light shooting, candid portraits, abstract shots, and food/product shots the Sigma would be preferable. For all around usage, landscape, and architecture shots the FA35 would be better suited.
The FA35 would function as an all-purpose walk around, whereas the Sigma 30mm would be more of a specialist lens for when you want extreme separation subject and background. IMO, the Sigma would serve better as a specialist lens to supplement something in the same focal range, i.e. the DA 16-45 f/4, where as the FA35 would be better if you're planning on going the all prime route.