Before choosing a metric, define the decision it should help you make. Registration numbers show interest; access records show actual attendance. Comparing them helps identify no-shows and adjust outreach, capacity and resources.
Engagement means looking at the interactions that matter for your experience: activity participation, survey responses or connections made. Higher volume does not always mean a better result: context and the quality of those interactions matter.
To discuss ROI, relate revenue or financial benefits to costs. To assess ROE, define experience objectives and how to observe them. The quality of conclusions depends on the available data and the scope configured for the event.