Simple Crossover
Step function and lambda ~ SSM automation and SSM RunCommand
SSM Automation is designed to work more on your EC2 instances, doing stuff like patching, rebooting servers, upgrading AMIs, etc, with some resource actions tailored to that job. I never actually used it, but it seems to be based on the same Step Functions engine or something really similar.
Step Functions is the OG, it will orchestrate your states, you can bring info from one state to the other, have state machines running for a year, run your state machine behind an API gateway, etc.
I would user SSM Automation if I can benefit from some of the SSM specific actions:
https://docs.aws.amazon.com/systems-manager/latest/userguide/automation-actions.html
Step Functions has some cool tricks as well, like distributed map, parallel execution, etc.
- A Reddit user