Cassandra was designed for fast access of data and makes compromises for writes.
CockroachDB was architected for complex, high performant distributed writes and delivers scale-out read capability.
Cassandra queries require you to understand partitioning of the database.
CockroachDB delivers simple relational SQL transactions and obscures complexity away from developers.
Cassandra implements a variant of SQL called CQL, which may limit application functionality.
CockroachDB is wire-compatible with PostgreSQL and provides a familiar, easy interface for developers.