In our project we used both methods.
We used the automation for repeating test cases. Especially for Smoke Tests and for normal Regression test suites. E.g. a user story has been created, then we adapted the user story in our test automation procedure. This means that our test automation suite has been enlarged dramatically but that was ok. But you have to make sure, that you always have the updated test suites / cases.
In case of e.g. layout testing or testing in different languages we used exploratory testing. Here we invited people who are e.g. native speaker to do some kind of "user journey test". Means that a application should be tested in combination with mobile app versions. For example let's say we want to do a purchase order at amazon. And I want to get paypack points, so I take an item (e.g. playstation) put it in shopping cart and in order to get paypack points I use my mobile phone and continue testing on my real device.
So in short we can say that we used exploratory testing mainly for:
- testing different layouts
- testing in different languages (here we invited native speakers e.g students we got about 30 languages to test)
- with the usage of real devices we also used exploratory testing
- user story testing with different heuristics& data attacks
So regarding the last point, we created automatic test cases for user stories AND we tested user stories with different heuristics. Because exploratory testing is about "Simultaneously-designing and executing test to learn about the system, using your insigths from the last experiment to inform the next." (see link: Exploratory testing definition
So in our case exploratory testing was absolutely required. Once we found out that the exploratory testing was working smooth we created automatic test cases (if required).
Here are some inputs for exploratory testing links:
Satisfice Link Exploratory testing
Quality trees Software Heuristics
I would use both combinations to receive a better test coverage. Because as already said, automatic testing is just testing step by step from scripts.