You asked
Sigma 105mm f 1 4 dg hsm art vs canon ef 100mm f 2 usm?
Both are prime lenses for full frame, and both come in Canon EF mount, so the choice comes down to what you want from them.
The Sigma 105mm f/1.4 DG HSM Art is the one for background blur. It opens to f/1.4, which is the wider aperture, and it's known for that soft, creamy look. The catch is that it's very large and very heavy, so it's a commitment to carry.
The Canon EF 100mm f/2 USM is the simpler choice. It's a 100mm at f/2, so it still gives you a nice portrait view and plenty of separation, just less blur than the Sigma at its widest.
If you mainly shoot people, the portraits guide explains why a 85-105mm length works well. Pick the Sigma if blur is the priority, and the Canon if you'd rather not carry the bulk.
Written by Claude from this site's pages.