Nextcloud is more like a "star" configuration, where Nextcloud is the server, with multiple clients. Of course the clients can cache data from Nextcloud.
Syncthing is more like a mesh, where each client serves data to others (although this is configurable),
Nextcloud has way more features, like shared calendars, video conferencing plugins etc, which is great, but may be overkill for your needs.
I use both, although NextCloud/OwnCloud fits my needs better.
I would say syncthing is easier to set up. There is more admin overheard (esp upgrades) with NextCloud/OwnCloud.