Between the two I recommend OnlyOffice because it has better export compatibility, a PDF editor, seamless interchange between their desktop, mobile, and web versions, and a nicer UI. You’d only need LibreOffice if you’re doing very advanced document formatting or very niche scientific or financial formulas. LibreOffice has been around for a lot longer than OnlyOffice so its got more edge use cases figured out. Onlyoffice is more modern and IMO feels like a smoother experience with the UI.
The guy who founded owncloud (Frank), sold it to american venture capitalists. When Frank didn’t like how the VCs were planning to enshittify Owncloud, he forked the project and moved it back to Germany from Boston. The American VCs saw this, realized they couldn’t exploit the software anymore, so they sold Owncloud to a group of German businessmen. Ironically for Frank there are now two german owned non-VC backed cloud software based on the same code. But yeah they’re taking different strategic paths for growth.