Zoom was founded in 2011, and they've had incredible growth since launching. Zoom is mainly known as a meetings platform, not a webinar platform. It's reliable, stable, and has great stream quality, and these features work really well for meetings. However, as a webinar platform, it's lacking quite a few marketing features, deep integrations, and it also requires a download for all attendees.
ON24 was built to be an engagement platform for prospects and customers in 1998. While the platform has been around for quite a while, it still maintains many of the outdated interface elements throughout the product; this results in a poor user and attendee experience. ON24 has a ton of great marketing features, but in many ways this also makes the application seem incredibly "busy" and hard to learn. It's far from a simple experience.
ON24 mainly targets Fortune 500 companies, and their prices are significantly higher than many platforms on the market. In order to even see their pricing, you'll need to get in touch with their sales team, as their pricing is not public.