Why OSX symlink cannot Relative PATHs?
This isn't about relative paths. Relative paths work fine.

If what you're trying to do here is create a symlink from Dropbox to BTsync, the problem is that Dropbox already exists. This means the symlink is created inside the Dropbox folder, named BTSync. Delete Dropbox and try again.

If you really meant to create a symlink inside the Dropbox folder, then both these commands work identically.