Dotcloud is built on top of EC2 to provide super painless configuration and management. If you want to manage your whole stack, EC2 is not a bad way to go, but for early testing and deployment with modest hosting needs, it's a cheap way to avoid a lot of sysadmin work. Particularly if you haven't done it a whole bunch of times before. There are a whole bunch of folks that provide this type of service, it's called "Platform as a Service".