Aurora PostgreSQL and Citus are pretty different. The big draw of the former is that it provides PostgreSQL as a managed service on top of a cloud-optimized flash storage layer that facilitates high availability, durability, and low-latency read replicas for PostgreSQL workloads that fit on a single machine. The latter provides a product (or managed service in the form of Citus Cloud) that performs transparent sharding that allows for scaling relational workloads beyond the limits of a single machine. Most workloads will favor one or the other of those scenarios pretty clearly, either by not stressing the capabilities of a single machine or by being easily shard-able.