Others have replied with basically the same thing but replication has been in MySQL for a long time. Just last night I set up replication for a single MySQL server to a new 3-node Galera cluster then switched over the Galera cluster as the primary with only a few seconds of downtime. The downtime could have been entirely eliminated if the standalone MySQL server had binary logging enabled already.
Galera cluster is an awesome multi-master solution and I've had nothing but excellent experience with it in production environments. We currently typically direct writes at a single node and spread reads in round-robin to all three nodes using an HAProxy front-end to the cluster, but there is nothing preventing us from spreading writes across the cluster if it was needed. If the current write node goes down there is only a couple seconds of interruption as a new write node is elected and the application reconnects. Furthermore, if a node goes down rejoining is completely automatic once it starts back up with no slowdown in reads and writes. I don't have near as much PostgreSQL experience but replication and failover have been serious pain points from an operations stand-point when I have had to use it. I appreciate the end-user features of PostgreSQL, but operationally MySQL/MariaDB (plus Galera cluster) are miles ahead of anything I've seen for PostgreSQL.
Galera cluster is an awesome multi-master solution and I've had nothing but excellent experience with it in production environments. We currently typically direct writes at a single node and spread reads in round-robin to all three nodes using an HAProxy front-end to the cluster, but there is nothing preventing us from spreading writes across the cluster if it was needed. If the current write node goes down there is only a couple seconds of interruption as a new write node is elected and the application reconnects. Furthermore, if a node goes down rejoining is completely automatic once it starts back up with no slowdown in reads and writes. I don't have near as much PostgreSQL experience but replication and failover have been serious pain points from an operations stand-point when I have had to use it. I appreciate the end-user features of PostgreSQL, but operationally MySQL/MariaDB (plus Galera cluster) are miles ahead of anything I've seen for PostgreSQL.