syncthing is great software, i use it all the time, never used owncloud personally

but i guess the latter is more geared toward single file server usage; you might want to replicate some data on every device you use it on, as syncthing does, but not all of it especially large stuff like directories with tons of images and video 

 for example, owncloud supports things like personal music streaming which is quite cool, if you don't want to use spotify but also  wouldn't want your entire music collection on every device