Starting with World War Z, the first thing to say is that it's a well conceived work, which takes in the sheer scale and suffering of this kind of disaster as it takes apart the human world piece by piece. Brook's decision to present the work not in real time but as a series of accounts from people across the world is a sensible one, it allows for the worldwide nature of the disaster to be played out without becoming bogged down in the minutiae of the survival. It also allows Brooks to maintain a level tone overall without worrying too much about giving different characters hugely developed voices, they're around for a chapter at most and then gone, and much of their accounts is reported speech.
The narrative takes us carefully through the apocalypse from its Chinese origins, as Brooks quite cunningly echoes the SARS outbreak, to the final almost victory the living attain over the undead. Like many post apocalypse novels and stories the genie can't quite be put back into the bottle and humanity is forced to rebuild around the fact that the zombies are still out there and that new people will be infected, and the living are forced to adapt as a concequence, here building special communities that are more defensible and taking advantage of the slow moving but ultimately flammable nature of their foes. One of the most interesting aspects of the novel is the way that military tactics used to try and defeat the zombies change from shock and awe to almost a World War 1 set up, with soldiers waiting for the zombies to advance and methodically picking them off with a series of kill shots.
Where the novel falls down is that American-centric nature of the writing, this makes the parts of the novel based in the USA very strong indeed and its at its best when discussing the fall out in North America, but the rest of the world seems to often be reduced to a series of stereotypes that do little to satisfy the reader. Britain slips back to castle reliant feudalism, the Japan section focuses on someone becoming almost a wandering monk and Russia slides back into an oppressive regime, this time dominated by the Orthodox Church.
Zombie Apocalypse in contrast concerns itself only with the UK and at first sight seems much stronger. The novel's conceit, a group of horror and science fiction novelists building a mosaic of the UK sliding into madness and destruction at the hands of an army of zombies in a vaguely dystopian future where the government is throwing money into a Festival of Britain to try to turn the economy around. In doing so they disturb something that should stay buried and catastrophe, as always, follows. Again the novel tracks through the development of the apocalypse from before the point that the threat becomes imminent to the conclusion usually through a sort of "found footage" device in the form of tweets, blog entries, mobile phone messages etc. This is a nice way to do it but does bring up the problem of it often not being very emotive. In fact a lot of the novel feels as if it's actually told in the same voice and not a great deal of it is that memorable. The honourable exceptions to this are Sarah Pinborough's pieces, a diary written by a teenage girl, and Kim Newman's hodge podge of reports and emails that somehow strike enough of a chord to stand out, though this may be largely to do with the fact that the survival plan initially hatched is "run like fuck" or words to that effect which chimes so neatly with the Freedom of Information requests in Leicester and Bristol in the last year.
I think, that because so much of the novel's pieces end the same way, with the people dying at the hands of the undead that it's very hard to feel that anything significant happens and it leeches out much of the detail, leaving only a sort of fog where things have definitely happened but the specifics are lost, despite there being stronger plotting than in World War Z and with Zombie Apocalypse being a more ambitious project.
Both novels have issues, one that's common to both of them is that after a while it's hard to keep zombies interesting, they're a pretty limited monster and one that has been, perhaps over exposed. I'm not sure that either novel really manages to keep them interesting, but World War Z just gains an edge because it's so much more sweeping and in many ways less about the nitty gritty business of surviving.