i simply do not see the point of getting the sigma 30mm at all. there are other lenses that are comparative or better at the price range. Also, the sigma is not compatible to a full frame, so when you upgrade or decide to get a second body, this lens won't be quite useful.
I'd get either the canon 28mm f/1.8 or nikon 35mm f/1.8. both offer great image quality and are both compatible with full frames. the 35mm focal length (or approximate) is very versatile as it is a wide standard; great for street photography while putting your subject into context. Also, the 35mm can also be a 50(ish)mm on a crop sensor. the 50mm on a crop sensor will be too much of a telephoto, which won't be useful in most scenarios. the 50mm on a full frame, however, is pretty much regarded as the "standard" lens.
overall, as i had mentioned earlier, it depends on what camera you have, what you are shooting, and what you will be planning on purchasing later on.