BTSync: is syncing symlinks, which is what I need* for easy distributing my files. (but thats the one and only reason)
*need: its too complicated/userunfriendly/inconvenient with syncthing to configure so many ignore lists all the time. In BTSync, I just can create a link into a synced folder and have all the files synced immediately. Booom.
Dropbox: -because its easy to share files/folders via a link. -the version history saved my life two times! -There are apps for my favorite smartphone/computer, which give me the comfort of using data from the dropbox almost instantly. (iKeepass, for example)