I'm not aware of any such comparison but to still give you some answer, I'd say that both products have a bit different focus. Prefect (and Orion) is primarily a workflow orchestration tool focused on eliminating negative engineering (i.e. wasted time). Prefect makes it easy to turn any Python script into a workflow and saves your valuable engineering time thanks to features such as retries, caching, results, subflows, passing data between tasks, managing execution states in an intelligent way (e.g., via automations, triggers and state handlers), and many more.
Afaik, Dagster's direction is different as it focuses more on data assets (i.e., what your tasks are producing) than orchestrating tasks and flows and managing the execution states. I think the only way to get an unbiased answer here is to give both products a try and decide which API and architecture fit your use case better.