This is basically your choice. They both satisfy the standards, so the homework ecosystem will run on either of them. If you want a safe bet, then go for HotSpot.
A bit of context. HotSpot was originaly developed by Oracle and it was the go to JVM years ago. Since then, many new implementations were released, one of them is OpenJ9. OpenJ9 was originally developed by IBM, but now it is open-source. It supposedly consumes less resources, and it is a bit faster. But see the comparison in this Medium article for yourself.