Swiper is the more common of the two in our index: it runs on 7,603 websites against 190 for ScrollMagic, and 57 sites use both. Counts come from real scans, not vendor claims.
ScrollMagic is a jQuery plugin which essentially lets you use the scrollbar like a playback scrub control.
Swiper is a JavaScript library that creates modern touch sliders with hardware-accelerated transitions.
57 sites run both ScrollMagic and Swiper. That is 30% of the sites using ScrollMagic, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.