Hey, author here. Currently you can sign in with GitHub/Google to sync History, Collections and Environment variables. But syncing with a Git repo isn't available yet. This is one of the priority request in our roadmap and internally we're working on this feature. There's an active issue to track its progress.
There's no list of comparison. In terms of features Postman has lot of features. Hoppscotch (as of now) only has essential features. Hoppscotch is a community driven project, new features are being added on every week.
Hoppscotch is a web app, you can spin up a API test from your smartphone / tablet on the go. It's an open source project, you can make infinite collections, environments and sync them across devices. Apart from REST APIs, Hoppscotch has WebSocket, GraphQL, SocketIO, SSE, MQTT, API Documentation generator etc.