Should we use RDS or Xeround?

We use Amazon S3 and CDN. Our app runs on Heroku. We are thinking of using a DBaaS like RDS or Xeround.

Because we are already using S3 and CDN from Amazon, should we use RDS? Since all technologies are running on the same host (Amazon), will there be any performance gain, rather than using something that is of third-party (Xeround)?

Or it shouldn't matter?