This blog has a good summary of the architectural difference between Collabora and ONLYOFFICE — Collabora runs more on the server side so needs more resources:
The way Collabora Online works is:
An embedded version of Libreoffice runs on the server. It reads the document, then ‘streams’ the rendered document as image tiles to the browser client, which shows it to the user. The browser client does some of the menu’s and lots of smart things like showing the cursor, other users, text selection etc, but many other components like pop-up menu’s and sidebars are also streamed from the back-end, giving relatively good feature parity with LibreOffice. This strategy is responsible for giving LibreOffice, for example, desktop-level table style editing, better than any other online office solution.The way ONLYOFFICE works is:
The document is converted on the server to a JSON file which is streamed to the browser client. The browser client is the full office suite, editing the document. Once done, it sends back the JSON and the server merges and exports it back to a file. A fully html5 canvas based front-end means a relatively pretty user interface and any javascript dev can go hacking.
I’d suggest that if you use LibreOffice on the desktop then Collabora probably makes most sense and if you use Microsoft Office then ONLYOFFICE probably makes more sense.
At the moment Webarchitects only offers a stand alone ONLYOFFICE server for all our Nextcloud clients (I’ve found that this works well when installed using Debian packaging rather than running it in Docker, which was a nightmare) but I intend to have a Collabora option in place when I find time.