I have both books, and can offer kudos and criticisms for each..
Kaufman's book, "Lives of North American Birds", while dealing with each bird species in particular, is the less effective of the two, in my opinion. For example, when discussing eating habits, the book almost invariably states, "eats insects with some seeds or fruits" (switching the order around from time to time depending on the bird). Well, that's the food of choice for 90% of birds, and seems a waste of space to repeat that nearly 700 times. They could've used that space to find something more unique about that particular bird. The problem is, this book was originally published as computer software (before eBooks became vogue), and when porting over to book form it brought over much redundant data. Still, it's really the only book of its type available for North America - unless you want the 18,000 page Birds of North American published by Cornell and the AOU, for $2000.00 (I wish!) - so it's worth having for that alone. But I find myself referring to it less and less as time goes on. It doesn't really give me anything that I can't get from Cornell's online field guide (which is 10 years more up to date).
Sibley's "Guide to Bird Life and Behavior" on the other hand, seems to understand the limitation of a bound published book, and does an excellent job of printing pertinent facts and useful information. It's much more of an introduction to ornithology course, spending the first 100 or so pages on birds in general, covering basics like anatomy, behavior, habitat and conservation, and then going into more details about each North American bird family in the subsequent chapters (each bird family has its own chapter). Of course, as has been noted, since they don't get down to the species level, they can only cover characteristics that apply to the whole family in question, but since there are many things that make each family unique from other families, there is still plenty to cover. And they do take the opportunity to talk about the unusual instances that separate one species from the rest of its family, so it's not just family-level discussions.