I received a TPB version of the Black Mirror for my birthday last year, and found the story...pretty standard. I dunno...maybe I'm dumb or something, but I couldn't really see why people found it so amazing. It's a decent story, and I get the themes that Snyder wanted to explore, but I really think his n52 work on Batman has been better.
I admit thay DoTF suffered from a pretty silly beginning (Joker being let loose on the GCPD, and that stupid encounter with Batman on the bridge), but the way Snyder portrayed the Batman/Joker relationship was masterful. The taunting. The waiting. The hunt, and finally the 'death' of the family.
To be clear, let me say that neither piece is my favourite from Snyder (Batman and anything else), but given the above choices, I vote DoTF