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.
I use Amazon DynamoDB because it integrates seamlessly with other AWS SaaS solutions and if cost is the primary concern early on, then this will be a better choice when compared to AWS RDS or any other solution that requires the creation of a HA cluster of IaaS components that will cost money just for being there, the costs not being influenced primarily by usage.