Promscale PM here :)
Promscale is the open source observability backend for metrics and traces powered by SQL.
Whereas Mimir/Cortex is designed only for metrics.
Key differences:
1. Promscale is light in architecture as all you need is Promscale connector + TimescaleDB to store and analyse metrics, traces where as Cortex comes with highly scalable micro-services architecture this requires deploying 10's of services like ingestor, distributor, querier, etc.
2. Promscale offers storage for metrics, traces and logs (in future). One system for all observability data. whereas the Mimir/Cortex is purpose built for metrics.
3. Promscale supports querying the metrics using PromQL, SQL and traces using Jaeger query and SQL. whereas in Cortex/Mimir all you can use is PromQL for metrics querying.
4. The Observability data in Cortex/Mimir is stored in object store like S3, GCS whereas in Promscale the data is stored in relational database i.e. TimescaleDB. This means that Promscale can support more complex analytics via SQL but Cortex is better for horizontal scalability at really large scales.
5. Promscale offers per metric retention, whereas Cortex/Mimir offers a global retention policy across the metrics.
I hope this answers your question!