The verdict
If webinars are a lead source and you want to know which attendees to call, pick Demio. The engagement tracking is what you are paying for.
If your team already uses Zoom every day, your audience knows it, and you need big broadcasts such as town halls or company announcements, pick Zoom Webinars. Your hosts already know the interface and there is no second tool to learn.
This one truly depends. Zoom Webinars is priced per host on top of a Zoom licence, so check the total before you assume it is the cheaper route. If you are not on Zoom already, I'd start with Demio. If you are, run one test in each and compare what reaches your CRM.