What's the difference between a space blanket and a reusable bivy?

Space blankets are single-sheet mylar foils, typically a few dollars each, very compact, and prone to tearing after one or two uses. Reusable bivies like the SOL Escape line use thicker, more durable fabric sewn into a sack shape, cost much more, and can be packed back into a stuff sack. Reviewers in r/Backcountry and r/Ultralight broadly agree the bivy form factor traps heat better than a flat blanket.