Shot in RAW and converted to JPEG in DPP with no processing. Tamron=T, Sigma 30mm f1.4=30mm, Sigma 28mm f1.8=28mm.

Fist thing noticable is that the 30mm has a lot more light falloff wide open at f1.4 than does 28mm at f1.8. Not a surprise since the 28mm is made for FF. The light fall off is noticable till about f2.8 on the 30mm. I'm surprised that the Tamron is about as sharp as the other two at f2.8 (center and corner). The 30mm also has a touch more corner softness till about f4. With processing, I don't think the difference is noticable. On the wall shots, the 28mm seems to have more CA than the others.

Flare test shows the most CA and worst contrast with the 30mm, however, it's also faster at f1.4. Stopped down to f4, the 30mm still has the most CA and worst contrast. Tamron and the 28mm are much more similar with Tamron having a touch less CA and a touch more contrast.

In the Bokeh test, the 28mm wins easily due to a fast speed and a very close min. focus distance, throwing the background really out of focus. The 30mm is next with it's wide aperture, followed by the Tamron. The 30mm and the 28mm seems to have a very sharp catchlight while the Tamron's is a bit soft, possibly due to focus being just left of the catchlight.

Sigma 28mm f1.8 is a sleeper lens but with it's close focusing ability, if you can tolerate the slower noisier AF that hunts more in low light, will reward you with composition that you cannot obtain with the other two lenses by it's very small min. focus distance/magnification.