I am using Collabora at this moment, but would like to switch to OnlyOffice. There are some features missing (like Conditional Formatting) which I do use alot. Because of that I am using Collabora.
In my opinion Collabora works nice, but does have some features missing, which I assume would be on the roadmap at some point.
OnlyOffice is better in regards to be more compatible with MS Office documents, making it easier to integrate with existing users which are used to MS Office.
Once OnlyOffice 4.5 comes out I will plan to switch over to it, unless Collabora catches up with some features.