It looks like Postwoman is is more of a simple utility right now. Perhaps it will grow to something more full-featured in the future but that seems to be the main differentiator right now. Insomnia, for example, supports environment variables, a GraphQL IDE, plugins, and many different authentication formats.

As the creator of Insomnia, I love seeing new projects like this and am really excited to see what Postwoman grows into. It looks awesome, is open source, and there's always room for more developer tools! :)