John McCormack identifies some differences between running SQL Server in EC2 versus RDS on Amazon Web Services:
How do I run SQL Server on AWS?
Running SQL Server on AWS can be done in 2 ways.
Relation Database Service (RDS): AWS’s managed solution where some of the administration (maintenance, backups and patching) is handled for you.
EC2: Your very own virtual machine in the cloud. With EC2, you manage SQL Server, just like you would do on-premises. This gives you full control over your SQL instance.
Click through for the comparison.