Both are good. Both are free to start. The winner is decided by one question about your files - not by a feature list.
Every Nextcloud admin meets this fork: the two serious in-browser editors are OnlyOffice and Collabora. Marketing pages will not decide it for you honestly. This page will - starting with the one fact about file formats that outweighs every feature comparison.
Both products edit documents inside Nextcloud, support co-editing, and have free self-hosted editions. The difference that decides daily happiness is format. OnlyOffice uses Microsoft OOXML (docx, xlsx, pptx) as its native format - files round-trip to Microsoft Office users with the least conversion surprise. Collabora is built on LibreOffice and uses OpenDocument (odt, ods, odp) natively - it is superb with ODF and converts OOXML on the way in and out.
So the first question is not about features at all: which format do the files your office exchanges with the outside world live in? For most Indian offices trading documents with clients and government portals, that answer is docx and xlsx - which is the world OnlyOffice is native to.
OnlyOffice presents a ribbon interface very close to modern Microsoft Office - staff moving from MS Office feel at home within the hour, which quietly decides adoption. Collabora's interface follows the LibreOffice tradition of menus and toolbars; powerful and complete, but a small relearning step for an MS Office trained team. Neither is decoration: the editor the staff avoid is the editor that fails, however good its engine.
If your world is Microsoft formats and staff trained on MS Office, choose OnlyOffice - the reason this channel's setup uses it. If your world is OpenDocument, or you specifically value the LibreOffice engine and its format openness, choose Collabora. If you truly cannot decide, install one, run the team's five ugliest real documents through it, and let conversion fidelity on your files make the decision - not a feature table.
The video commits to OnlyOffice and builds it fully - this page is the honest case for why, and when the other door is the right one.
Watch Full VideoTechnically possible, but a poor idea in practice: two editors writing the same files invites format conversion ping-pong and doubles your maintenance and support. Pick one editor as the office standard and keep life simple.
Both handle normal office sheets well. For extreme files, test your own worst spreadsheet in each - engine behaviour on huge, formula-heavy xlsx files is exactly the kind of thing that differs in practice and cannot be settled by a specification table.
Both have genuinely free community editions used by large numbers of self-hosters. Paid tiers on both sides buy support, scaling and enterprise features. A small office can run either without a licence bill - the honest costs are the server and your admin time.