Apples and oranges. Syncthing is a selfhosted p2p syncing network and it's miles better for this. Nextcloud is a selfhosted "cloud platform" like Google Apps. How good it is for each use case varies greatly. Most often it is used for easy web/mobile access for files and photos.
I don't know which one is better, but considering file hosting, sync and web access, Nextcloud works very well. Regarding usability, as NC doesn't works like Dropbox it is a matter of taste, I prefer NC over Dropbox.
+1 for FolderSync, it works great. I haven't had a single synchronization issue or missing files whatsoever after using it for 2-3 years already. Note that I never had the need to handle conflicts (I don't modify files from two different devices at the same time).
