Can't you use RDS like any MySQL database? And since you're writing a standard WSGI app, it seems like there is very little lock-in. This seems to basically automate the provisioning of EC2 instances for the web app and the DB. and since you just have a standard app, you're free to move it around as needed.
Or is there something that I'm missing with the Elastic Beanstalk? (I've only used raw EC2)