I have never used the Sony 35 FE 35 f/1.8, but I do own the Sigma I 35 f/2. I tried the Samyang 35 f/1.8, and while I liked the compactness, it just wasn't as inspiring to use. That led me to look into the Sigma I series.
In regards to the advantages of the Sigma, I love the build quality, handling and aperture ring while still maintaining a compact form factor. I also like the colors, rendering and special sauce in the Sigma that make it a great package.
