It's more of a swiss army knife. It's also still pretty new - the aws documentation misses out on some things for it (like autoscaling) and it's still in development to some degree, but it means you have to remember one command with one auth, instead of a handful (s3cmd, boto, others I can't recall)
In terms of CLIs for Amazon S3, 'aws s3' is a collection of commands that are almost equivalent to 's3cmd' functionality, while 'aws s3api' is a collection of commands to interact directly to API using corresponding action names.
In my experience the performance of the new AWS CLI is roughly the same as s3cmd as long as you are using a version of s3cmd with multi-part upload included.
