I believe Heroku used EC2 under the hood before the Salesforce acquisition.
Whether postgres (vs MySQL), the polish and better ui/workflow of heroku is worth the premium over RDS depends on you.
I'm not sure if these postgres servers are. However one interesting note is that the "maximum database size" of the heroku server is 2TB, while RDS is 1TB.
I believe Heroku used EC2 under the hood before the Salesforce acquisition.
Whether postgres (vs MySQL), the polish and better ui/workflow of heroku is worth the premium over RDS depends on you.