Zoom vs Prime - Quality Difference & Sweet Spot

This post is a work in progress - right now we have some samples of the Canon 18-55 STM lens and the Sigma 35 f/1.4 Prime. The Canon costs ~$230, less when bought with the camera. The Sigma currently sells for $900.   The obvious difference between the two(besides price) is the wide open aperture the sigma is capable of- but when you stop down some- say f/4.5, the widest aperture available to the Canon 18-55 STM at 35mm - how do they compare.   I will have more samples coming soon.