The Kingdom. It was very interesting to read and see what would happen if the Mata Nui did die. I also liked how the end of the world actually united everyone rather than everything descending into total anarchy.
Dark Mirror is the weaker of the two, mainly because it’s done many times before. I’ve seen so many “what if the good guys turned EVIL” stories that I’m honestly sick of them now. Also, The Kingdom had better pacing, although that may be due it being a short story while Dark Mirror was a serial.