I’ve also set up both and in my experience Nextcloud is much much more complicated to set up but simpler to use and syncthing is pretty much the exact opposite.
In my case, a rather long time ago, it failed to reliably sync my files, had a super annoying web based UI, was a pain to get all my devices to talk to each other because because they had to join some sort of peer to peer network and authenticate with the earth other all three. It also didn’t have any working solution for mobile devices. Hopefully all of that’s fixed now because there’s no inherent reason it couldn’t work.