Vitess is a software suite that runs outside of MySQL while Citus runs within Postgres as an extension, so Vitess as more moving parts, but it also is better at handling more connections which has been a continual problem with PG in general.

Vitess supports more complicated sharding management (and requires more configuration) while Citus is simpler and lets you use all the flexibility of Postgres. Vitess has a more streamlined installation process (especially with Kubernetes) while Citus leaves it up to you to install the Postgres nodes.

Also note TiDB for a natively scalable MySQL-compatible database and CockroachDB for Postgres-compatible version.