They are completely different:
Postgres - Solid single-node relational database with SQL standards support, JSON, fulltext search, custom data types, ACID transactions, foreign data access, and more. HA/scaling through replication and some 3rd party offerings for automatic sharding, failover and multi-master if you need it. Great as the operational source-of-truth for your core data.
Cassandra - Distributed wide-column (basically advanced key/value) meant to run as a cluster of machines with native support for multiple data centers. Limited "SQL" support which is really only used for defining tables. Data access is very different (everything is an upsert or delete) with variable per-query consistency settings. Great if you need data spread globally, 100% availability, eventual consistency fast key/value, and some interesting data access patterns.
If you want high-performance Cassandra, look at ScyllaDB first. If you need something in between Cassandra and Postgres, look at CockroachDB.