For enterprise integration scenarios automated integration testing is more important than unit testing. Ideally though you should do both.

Unit testing with the self hosted framework will make it easier to run lots of tests and scenarios but what happens when you actually deploy your workflow. Will they still run as expected, what happens when your connect to real systems and many other considerations come into play.

Regarding SpecFlow testing for Logic Apps: I am a huge fan of the way this approach allows you to lower the total cost of ownership of your solution by creating self documenting behaviour driven integration tests. I think this approach will make your life so much easier