[namibj]

btdu extra (or expert?) mode with snapshots kinda does that: you can see what's only in the new version and not in a snapshot; and vice-versa. Also it offers attributing size to folders only for extends that aren't shared with a different folder (snapshots are essentially just special folders), to kinda get a diff between the two (stuff only present in the old snapshot is shown there; stuff only present in the new version is shown there).