I get both.
The Official Guide is better I think for first timers or those not familiar with the parks. It's easy to read, laid out well, has some good pictures and isn't overly complicated.
The UG is a lot more in depth. As such, it would confuse the heck out of someone who's never been. It's too commando. Plus, the book backtracks alot and can be rather confusing at times.
That being said, I enjoy reading it, as most of what they say I think is pretty accurate, as far as touring plans go anyways.