Those are very different.
The URL one requires it to be pointing at a git repository and treats that repository the same as it would a remote one, cloning it and checking out particular versions or revisions. The source you see in Xcode is managed by SwiftPM and will be “locked” and uneditable.
The path one just directly uses whatever is currently in the file system at that location. I think you can even use it that way without either package having a Git repository at all. You will be able to edit the source you see in Xcode directly.
(Caveat: I’m speaking in terms of Xcode 10.2, but the related documentation of SwiftPM looks like it’s still the same.)