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 same interface elements throughout the product. ON24 has a ton of great marketing features, but this also gives the product a bit of a learning curve.
ON24 mainly targets Fortune 500 companies, and their prices are hidden until you get in touch with their sales team, as their pricing is not public.