On APS-C, both the Canon 28mm f/1.8 and Sigma 30mm f/1.4 are more natural “normal” lenses than the 50mm, so the first decision is really field of view.

Compared with the Canon 50mm f/1.8, the Canon 28mm f/1.8 offers much better handling: USM autofocus, full-time manual focus, a better build, non-rotating front element, and closer focusing. Wide open, center sharpness is generally comparable, but the 28mm tends to show softer corners and more chromatic aberration, and the corners don’t fully catch up even stopped down. For portraits that usually matters less, since the edges are often out of focus anyway; for landscapes, it can be disappointing.

The Sigma 30mm f/1.4 is generally regarded as sharper in the center wide open than either Canon lens, even at f/1.4. Overall image quality between the Sigma 30mm and Canon 28mm is fairly similar, so your choice may come down to focal length, speed, and handling.

If you mainly shoot portraits on APS-C and want wider than 50mm, either can work well. If edge-to-edge performance matters, the 50mm remains stronger; if you want a wider everyday prime, the 28mm or 30mm is the better fit.