AWS Lambda vs AWS step function

Step functions are excellent at coordinating workflows that involve multiple predefined steps. It can do parallel tasks and error handling well. It mainly uses Lambda functions to perform each task.

Based on your use-case, step functions sound like a good fit. As far as pricing, it adds a very small additional charge on top of Lambdas. Based on your description, I doubt you'd even notice the additional cost. You'd need to evaluate that based on the number of "state transitions" you would be using. Of course, you'll also have to pay for your Lambda invocations.

Lambda and Step functions are like floors and steps to each floor. You cannot replace one with another.

Lambda is computing, steps functions take them to the desired step.

Youtube video explains very well: https://www.youtube.com/watch?v=Dh7h3lkpeP4

To the analogy again, you can have multiple computes (lambda) in a single floor before you pass it on the next floor.

One of the example is as shown below.

Usecase: https://john.soban.ski/transcribe-customer-service-voicemails-and-alert-on-keywords.html

Hope it helps.