Nextcloud and syncthing a 2 different things. While synching keeps your files synchronized between your devices, nextcloud is a cloud platforme which gives you access to your files on your server. The difference is actually very important even though a lot would argue that they are the same.
It really depends what type of technology you want/need. I would recommended syncthing for it's simplicity and robustness. It only does one thing but it does it very well !!
To give you an example, I have a syncthing sever which is a middleman for my Zotero database. Laptop <-> Server (syncthing) <-> desktop
. It's very important to keep it in sync for the integrity of the zotero database !
This kind of workflow wouldn't work with nextcloud or rather will most of the time corrupt my database.
Nextcloud works as a simple cloud platforme wich is also great (with it's up and downs) but works only as a cloud platforme to keep your files on your server.