Labyrinth is dark. That said, the Dark Crystal is...well...darker. When I was very little and my dad was in the military stationed in Germany, he took my mother and I with him. We brought three videotapes in our luggage: Labyrinth, The Dark Crystal, and a 6 hour long pirated copy of the Tracey Ullman show. I'd argue that Tracey was more well-suited for children than either of the other two movies. But...here we are! I watched them so often I can still recite them both in my head.
Labyrinth is the more lighthearted of the two (but is absolutely terrifying to a 4 year old lol) and its certainly far more universally popular. Also...Bowie...so, you can't go wrong!
But, if you're looking for an absolute masterpiece of puppetry: The Dark Crystal is 100% a work of art. There are no humans in the Dark Crystal and it can feel slow if you aren't really invested in watching and appreciating it for what it is...but if you ever get in the mood to be floored by what Jim Henson could do, (and can stand to be horrified) then give it a watch!