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.