They are essentially the same assignment type, but the difference is who uploads the work to Gradescope. The Exams assignment type is automatically configured for Instructors to upload the work. Whereas a Homework / Problem Set is automatically configured for students to upload the work to Gradescope. The two are interchangeable, so if for instance you start creating an Exam (Instructors upload) and then realise you should have created Homework / Problem Set (Students upload), you can simply edit the assignment and change the settings.