Rhasspy is a more powerful general-purpose application and GUI; voice2json is more like a library or micro-service that does exactly one thing: convert a speech waveform to JSON. They share some DNA though (same syntax for defining vocabulary).

I used voice2json to build a voice-controlled car audio player, it works amazingly well: https://github.com/lukifer/voicetunes

They share a lot of the same pieces, but voice2json is meant to work in Unix-style pipelines. Rhasspy has MQTT/HTTP/Websocket APIs instead.