The verdict
For a small marketing or sales team running lead-generation webinars or product demos, I'd pick Demio. It is quick to start, and the engagement data gives sales a clear next step. If you run larger, regular events for training or marketing, need payments or certification, or already use other GoTo products, pick GoTo Webinar.
GoTo Webinar has no clear self-serve price, so expect a sales conversation. Demio has a trial with limited attendees and session length, and its higher plans are priced per host. Either way, run one test session and check what reaches your CRM before you decide.