tl;dr I can't decide which I preferred, City of Death or Rose, but I can say both are fantastic and will be re-read sometime soon. The Christmas Invasion is good and fun, but nothing to write home about.
Reviews, in order of episode date:
City of Death, by James Goss
James Goss is probably my favorite DW writer (challenged only by Nev Fountain) and his Target novelization is a great example of his work. He toys with a Douglas Adams style - deadpan absurdist - which only fits as the episode was written in part by him. However, it isn't forced and it doesn't overshadow his main goal, which is to tell one of DW's most complex and entertaining stories.
The novelization is faithful to the episode, but it delves deeper into Scaroth's story, especially that of the modern fragment, who, at the episode's start, hasn't yet realized who he is or why he's really doing what he's doing. It's only as the mask comes off that he starts to remember, and we get to watch the transformation from arrogant wealthy human to true alien from the inside.
Of course, there's also the Fourth Doctor here and he's no less fun, but I think the hidden gem here is Romana. She appears infrequently in this story but her smug alienness is far more palpable in this novel than in the episode, and we get to grin right along with her as she deftly guides Duggan through this strange world he's fallen into.
Rose, by Russell T Davies
I like to think that Rose is what RTD would have done with the first episode of the returning show if he hadn't been constrained by budget and the need to make the new show relatable to general audiences who had no knowledge of the original. As in the episode, the Auton invasion and the arrival of the Doctor are seen through the eyes of Rose, the suddenly-unemployed wastrel shop girl, but RTD takes this opportunity to do what he does best: bring all of the characters to life.
Rose is true to her portrayal in the episode, a teenager who presents the front of confidence and skill while subconsciously realizing that she's going nowhere and doesn't have the strength of character to fight for anything - until the last moment, when all hope is lost. But she isn't the treasure here. RTD paints everyone in this story - even Bernie, the guy in the Henrik's basement that Rose was bringing the lottery money to - and not only gives them reasons for being where they were and doing what they were doing, but also follows them after the invasion is over. (Well, except for Bernie, because he was already dead when it started.) Clive's the big example. Now we know why he's obsessed with the Doctor, and why his wife Caroline supports him, even though she doesn't believe that any of it is true. We also get a full background on Mickey, as well as more about Jackie.
The story itself is expanded, by the benefit of hindsight as well as by the lack of restrictions. Clive's information on the Doctor now includes previous incarnations as well as future incarnations - though Rose got distracted by the photo of her Doctor fighting a pterodactyl as Clive wittered on about a tall man in a brown suit. The Auton invasion itself became global, rather than the limited-budget shopping mall attack that was implied in the episode. The book also includes references to other events that have occurred in the show since the first series that the characters would know about, most notably Rose's encounter on the Powell Estate with a drunk man standing in the shadows, but there are also a couple of other gems hidden in there.
All in all, this novelization was beautifully done, bringing out the original flavor of the episode and enhancing it with intricate characterizations and expansions on the original material.
The Christmas Invasion, by Jenny Colgan
The Christmas Invasion was a fine read, but now, after having read Rose, I'm glad I read TCI first. It is faithful to the episode and deftly written, but it isn't much more than that. JC does bring a spotlight on Daniel Llewellyn, the director of the Guinevere One program, and explores the budding interest between him and Sally (I can't remember her last name) as they meet for the first time at UNIT headquarters, but beyond that, there's not much new here. Much of the emphasis of the book is on Rose and her despair at being abandoned by the Doctor, just like in the episode. The one thing I didn't like, and this is a personal preference thing, is that the end of the book got a bit shippy, with both the Doctor and Rose wondering if the other didn't like them anymore. I'm not a romantic and it just didn't appeal to me. However, I suppose any novelization from Series 2 would have to have this kind of thing. I guess I've been warned. :)