It depends upon your needs.
If you know absolutely nothing about the parks, then get Birmbaums. It has the most detailed description of what things are, both rides, restaurants, and such. It also has a few tips on how to get around and such. It has a few reccomendations for some do not miss type things, but other than that it is weak on advice. It is very weak on advice to save money.
If you'd like some plans as to how to get around, what to do first, how to avoid long lines, etc and general reviews and ratings and such, the UG is tough to beat. It has time saving tips, and money saving tips. It really is a good resource. I personally don't like the commando park plans, so I do not follow them to the letter. But certain principles laid out in the book are invaluable even if one doesn't go the park commando route. But if you've never been there and don't have a clue as to what anything is, the descriptions of things are basic as much more space is devoted to ratings and stats and what rooms have the best view and the like.