The main difference is that workflows are written as code in a general purpose programming language. Java, Go, Javascript/Typescript and PHP are already supported. Python and .NET are under development. AWS Step Functions are using JSON to specify workflow logic. JSON is OK for very simple scenarios, but is not adequate for the majority real business use cases. The fun fact is that Step Functions are a thin layer on top of AWS SWF which is based on the same idea as Temporal.
Here is a more detailed answer from Temporal forum: https://community.temporal.io/t/why-use-temporal-over-a-comb...