I love that Jabref supports working with multiple libraries (having multiple open the same time, moving entries between). Best Zotero could do was restart with difference preference files (has that changed? haven't used it in some time).
And really like that Jabref syncing requires just syncing the library folder. Zotero syncing really nudges you to the paid plan. setting up webdav just isn't as simple and the list of supported providers isn't that long.
It really helped me that the backend is a plain bibtex file. I could resolve issues with it myself. I can also version libraries with git.
