I've used Postgres RDS and it was fine. One limitation though is that unlike MySQL RDS, you can't (yet?) configure it for streaming replication. You can tick a checkbox to run a "Multi-AZ" deployment with failover, which I assume uses streaming replication behind-the-scenes, but there is no way to access whatever slave instance AWS is running. So you can't use slave instances for horizontal scaling, only high availability. I haven't looked at Heroku Postgres in a while, so I don't remember whether they offer what AWS doesn't here. It doesn't sound like you're anywhere near the point of scaling to a read-only replica though.