The Twelve felt very different to The Passage. It felt to me to be more of a thriller than a piece of paranormal speculative fiction (or whatever else you might classify it as).
Also, where The Passage seemed to deal more with the experience of the individual in the aftermath of the virus, The Twelve dealt more with the station of the nation following the release of the virus. We see people spread across the country living in different outposts, attempting to establish themselves in a manner they can sustain in the face of the adversity they now face.
Where some of those new cities are facing their challenges with respect for the human race, there is another city being established and controlled by the 'red-eyes' where this is not the case. The Homeland, as it is called, is run by people who have infected themselves with the virus but have not flipped, and they see themselves controlling the world in conjunction with the virals. In this way, an interesting question is raised in The Twelve: how can the virals sustain themselves when they have almost eaten their entire food source? It's a question I am surprised I never really considered while I was reading The Passage. In The Twelve we see one solution imagined by humans who are willing to betray their own race.
The characters I came to know in The Passage had all gone in their separate directions, and yet I still felt attached to them all. I was invested in their lives and read with baited breath to see what had become of them now.
I do have to say I was rather frustrated that there was so much about The Twelve that I didn't understand.
*SPOILER ALERT* Why didn't the red-eyes flip? Why was Lucius Greer so attached to Amy, was he her familiar? What is this ship they were talking about? How was it that Amy came to be talking to Carter, as if in real life? How is it that Wolgast came back into the story as a viral? Why didn't Alicia flip and who is talking to her at the end of the story? My questions could go on and on. *END SPOILER*
I hope very much that these questions are answered in the third book. If not, I am going to need someone a lot smarter than me to explain it all to me.
Ultimately, the pace was fast and I couldn't stop reading. If I were to be honest with myself I think that a big part of why I couldn't put it down was my commitment to The Passage rather than the contents of this book, but I still loved every minute of it.