Much of our data model is relational, which makes MySQL or PostgreSQL (and family) fit the API's we need to build, in order to meet the needs of our customers.
Sometimes the flexibility of a NoSQL store like Amazon DynamoDB is very useful, but the lack of consistency really impacts usability and performance long-term, compared with viable alternatives. At our current scale, we've seen huge benefits from moving some of our tables out of Dynamo and doing more in SQL.
There will always be use cases for NoSQL and key-values stores, but if your model is well understood in your business/industry: relational databases are the way to go after finding product-market fit. Always understand the trade-offs (and a few intimate details) of any data store before you add to your company's stack!
Amazon RDS for PostgreSQL or Amazon DynamoDB? If you need to follow RDBS concepts choose Amazon RDS (PostgreSQL). This choice will guarantee a stable relational database with a great support for years from PostgreSQL Global Development Group. But, you need a great scalable NoSQL database. You can use Amazon DynamoDB. The most important thing about this is that you can use RESTful HTTP API to access data.