Much more in depth UI similar to Google drive kind of UI. Has a bunch of plugins to do other things too. Bookmarks being one of them. I personally use both they have similar syncing functions but work differently. Syncthing is nice for data just being on multiple devices where nextcloud is nice to have a UI and web site to go to anywhere. Nextcloud get the spousal approval much easier too.

It is much, much easier to setup than syncthing, uses a decent GUI interface that works well with Linux, Mac, Win, iOS & Android. Lots of additional features beyond file sync/sharing.