Perhaps the best way to see if I am Team Zombie or Team Unicorn is to assess each story. (Note to anyone wanting to rush out and get the book for a young person: there's a good bit of swearing and some mature themes: it's not the next thing to get your fan of Goosebumps books.) (Note to self: can I stack up colons like that?)
Zombies vs. Unicorns, Justine Larbalestier and Holly Black, editors (2010)
"Highest Justice," Garth Nix - middle-weight fairy tale (unicorns: +2)
"Love Will Tear Us Apart," Alaya Dawn Jackson - pretty good love story; why do I feel like I don't get the ending? (zombies: +2)
"Purity Test," Naomi Novik - after various bit of suspense and bloodshed, ends with the line "okay - you know what, just shut up and give me some more chocolate milk." (unicorns: +4)
"Bougainvillea," by Carrie Ryan - elaborate, sad, bloody (zombies: +1)
"A Thousand Flowers," Margo Lanagan - medieval, sad, upper-tier fairy tale (unicorns: +3)
"Children of the Revolution," by Maureen Johnson - hauntingly awesome; captures the college-aged woman's voice perfectly (zombies: +5)
"The Care and Feeding of Your Baby Killer Unicorn," Diana Peterfreund - freakin' awesome killer unicorn story, layered in with the a teen's struggle to know what her faith calls upon her to do (unicorns: +5)
"Inoculata," Scott Westerfeld - he's such a masterful describer of post-apocalyptic worlds and he captures/creates the language of young people so nicely that I'll give this one high marks even though bits are a lot like Ryan's The Forest of Hands and Teeth (zombies: +4)
"Princess Prettypants," Meg Cabot - this hysterical story is the only thing by Cabot I have really liked! I may have to give her books another chance. (unicorns: +5)
"Cold Hands," Cassandra Clare - heavy-duty fairy tale-with-implied-lesson. Damn. (zombies: +4)
"The Third Virgin," Kathleen Duey - poignant; wished it had more depth (unicorns: +3)
"Prom Night," Libba Bray - while I love her off-the-wall humor: an ugly prom dress is an "unholy union of Hot Topic and mother-of-the-bride," another bleak zombie story is another bleak zombie story. Or were those fireworks a symbol of hope? (zombies: +3)
Looks like Team Unicorn for me! It strikes me that there's more room for variation in world-building for unicorns than for zombies, but great writing can take the day.
See also: Tor's interview with the cover artist, Josh Cochran.