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.

Yeah, s3cmd is for interacting with s3.

aws-cli lets you interact with 23 different AWS services (it's really awesome).

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.

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)

I did a drop in replacement of s3cmd by aws s3 and the end result was much better performance, no issues.

Also the 's3cmd sync' and 'aws s3 sync' commands don't have exactly the same heuristic so switching caused a one-time resync of everything.