I read Ficciones, recently, so I don't have much to add. The majority of the short stories are the same, but it was a pleasure to read through them again. The second time helped me grasp some concepts that slipped through the first time. Borges is incredibly intelligent and well-read, and it can be difficult to wade through philosophical musings, but taking the time to slowly read through makes the read more satisfying.
The essays at the end gave some background to Borges's creativity. He took all of his philosophy and made fiction out of it, which makes me wonder how long he labored with each of these short stories, some only a few pages long. It makes the stories seem less bizarre after understanding his philosophical influences.
The additional stories in Labyrinths that were not in Ficciones, and vice versa, don't add much to the magic, as his best stories are in both. Reading one or the other would give the same overview, yet Labyrinths has the additional essays and parables, whereas Ficciones only has short stories.