I think it depends upon what you want out of your guide book. I heard great things about so many of the different books that I had 6 different guide books and borrowed 5 more from the library; fodor's WDW for dummies, WDW on a budget, Birnbaums (for kids & adults), passporter, UG, UG w/kids, plus a few more (What can I say I read a book a day, sometimes two books.) Anyway, of the books I read all had much the same information but it was all presented in different formats and different depth with a different goal what the author wanted to teach you about WDW.
Both Unofficial Guide and Passporter are excellent books.
If you want concise but thorough reviews of the different rides, hotels and restaurants as well as colorful maps and you don't want to do a lot of reading then this is the book for you. It's quick and easy to find phone numbers, summaries about various activities and events. Drawbacks - basically no information on anything in Orlando area besides the WDW resort so if you plan on hitting Seaworld or Universal you need to get a different book. But the reviews are just that concise, they don't give info on things like travel times between resorts (of course you have the DIS for that), detailed reviews on noise levels in the hotels. And it was no help in deciding on-site or a house.
UG has all the details and dirt on everything WDW with great info like travel times between hotels and parks by every available method of transport. It also gives great indepth reviews of the restaurants, hotels, rides, the dining plan. It was an excellent resource for helping me to decide did I want to stay on-site or rent a house and then did I want the dining plan or not. And because of this book I knew I HAD to be on the phone first thing in AM on the day my window opened to get my character meal ADRs. It was also a great resource for finding online info; in fact the book was how I found the DIS. Plus there's the touring plans, which even though I didn't follow them to the letter while in the parks they really helped me plot my days which I needed for my middle child. But sometimes UG gives a little too much detail. I'm not going to decide what hotel I'm going to stay at based on the plushness of the mattress and I certainly don't need to know how they determined the comfort level of the pillows. Plus, if you are going to WDW and know that you are staying on-site and not going to Seaworld or Universal then there is a lot of extraneous info.
Finally, if you have kids, even tho the book is very biased, invest in (or borrow from the library) the Birnbaum's Guide for Kids. It was a great resource for my kids to read about and see photos of the various rides. Plus the ratings of scary, dark, etc are much easier to understand for kids than those in other books.
ETA - The passporter is extremely well organized, finding information is very very easy. This is not the case with UG where info about one thing can be spread across multiple chapters with a sentence here and a paragraph there.