I think it would be tough to compare. There are client libraries for several languages, but the project is mostly a SQL API to the queue operations like send, read, archive, delete using the same semantics as SQS/RSMQ.
Any language that can connect to Postgres can use PGMQ, whereas it seems River is Go only?
Any language that can connect to Postgres can use PGMQ, whereas it seems River is Go only?