Both OnlyOffice and LibreOffice (Writer) would work. The main differences are that OnlyOffice has all the tools (documents, presentations, and spreadsheets) while LibreOffice separates them. LibreOffice also has an additional two tools: Draw (kind of like Adobe Illustrator or Inkscape) and Base (database stuff).

Generally documents that are exported to and from MS Office will be better when working with OnlyOffice. If you aren’t doing a bunch of formatting that is absolutely critical (or no formatting at all), I wouldn’t worry about using LibreOffice. LibreOffice, for me, works well, but sometimes page break can happen a line or two earlier/later depending on how images, tables, headings, etc. are rendered, which is slightly annoying. In “Impress” (presentations), whenever I made one object transparent, ALL objects became transparent when viewing from MS Office, which was strange. That’s my experience, and it seems that other people have had similar formatting problems with LibreOffice too.

Another thing that’s different is that, on Windows, LibreOffice doesn’t look very nice. On Linux, it looks fine (I selected “Tabbed”), but on Windows, I’m not sure if they’re using some other graphical package or something, but the top toolbar is all squished together with no wiggle room, even if you select “Tabbed”. One other thing, LibreOffice uses Qt so it will work with those themes in Linux (e.g. In KDE Plasma, which is what I run, LibreOffice will match with the system theme!)

For a Linux-specific thing, it seems that LibreOffice doesn’t really like Wayland much, and was chugging and running at a jogging snail’s pace (certainly not a fast snail, this one), so I had to manually set the shortcut to run LibreOffice in XWayland or something like that anyways.

One thing I like about LibreOffice is that there’s a few more plugins available than OnlyOffice. For example, LanguageTool has a LibreOffice plugin but don’t think there’s one for OnlyOffice. LibreOffice also has like a bazillion different settings to change how everything is rendered, you can change the order of the items in every toolbar and menu, I think Writer has a basic Java IDE built in too? Basically, LibreOffice is more customisable and configurable than OnlyOffice, and by a lot. OnlyOffice tries to be a little simpler than LibreOffice and I guess is more “dumbed down” with fewer buttons and settings?

TLDR: LibreOffice has five separate programs, OnlyOffice has three in one. Files exported from OnlyOffice generally render with correct formatting on MS Office (and vice versa), the same may not be true for LibreOffice depending on the content of your documents. LibreOffice Windows isn’t nice (but looks better on Linux), LibreOffice doesn’t play nice with Wayland, LibreOffice has more plugins, options, etc.