I'm a librarian so I checked out every book my library system had on WDW! I liked the Unofficial Guide the best. The UG is very through, well-researched, detailed, objective, and even a bit humorous. The author is very honest, which some people take as being critical of Disney. I did not feel this way; rather, I felt like he was just being realistic. The UG also incorporates reader's opinions so you know you're getting varied viewpoints. I like the suggested touring plans the UG offers. This is a very good book for a person who likes to plan in-depth.
Birnbaum's guide was nice but I found it to lack objectivity and I suspect this is due to it being the "official" Disney sanctioned guide. I did like Birnbaum's for attraction descriptions and it does have pictures. This is a good book for a person who wants a general overview and doesn't need to plan every second of his or her trip.
I agree with the previous poster who said that each book makes up for what the other lacks. However, if I had to purchase just one, it would be the UG. Birnbaum's is a nice supplement to the UG. Birnbaum also has a nice dining guide, but I think it is only for WDW. Dining info is covered very well in the UG. Again, I used the Birnbaum dining guide as a supplement.
My suggestion: go to the library and check out both books before you decide to purchase one (or both). If your library does not have either book in circulation, they would probably be willing to purchase it for their collection.
Good luck and hope that helps!