so what is better ??

I'm asking this because I saw a guide book for Oblivion when I went to Microplay but didnt bother picking it up, but it was on its side and had to be atleast 100-150 pages thick by looks of it

The book - which I have - has a fixed amount of data. It was compiled before the game was released and never updated, logically. The UESP has more information and more up to date information, since it's also being maintained by a large amount of people. There's probably nothing in the book that you can't find on the UESP. I'm not so sure about the other way around.. Regardless, the book is a nice product and cool to have.

I have the book, and I love it. I find it so useful to just flick to a page or look at the maps in it without having to fire up the laptop (I play on 360) and load the pages I want.
The book is fairly rare now too, and ones you can find tend to be quite expensive.
I like it just as a collector's item as well. I think the book is well worth owning.

The UESP wiki covers a few user-made major mods as well as the DLC's. I'd assume an "Official" guide would only cover the Bethesda content, and maybe not have all the DLC's depending on when it was published.

There should be a GotY version of the guide as well as the original, and the GotY version should have at least the Knights of the Nine and Shivering Isles DLC included.

The official guide, like other Beth official guides, is an excellent reference tool. The problem with it, as with most any RPG guide, is when it attempts to offer specific advice. This problem also applies to the UESP, though, or any other guide. Specific advice may not apply for your character concept(s) or play style. That is a common issue I find with any guide.

Also, note that the UESP wiki has its own flaws, of course. For example, the information on the UESP wiki about Speed and how it actually functions within the game, particularly between enemies and PCs, is incomplete, largely due to the very complex method and many factors that speed is dependent on ingame. It also doesn't help that using the console to create certain contexts may not be the same as playing a character from the beginning and developing the same contexts (e.g., scaling of characters winds up treated differently between custom races from the CS and simply using the setscale console command).

Basically, with a game as complex as TES games are, it is almost impossible to detail all information. Therefore, it's good to use multiple sources and check your own game experience.

I have the Prima Guide book, an amazing read even if you don't use it for the walkthroughs. One thing I did find handy was the book's character creation guide. Gives name suggestion for each race (One each for Male and Female) and tips on custom classes. UESP is a handy resource for stuff not covered in the book (RP Tips, Fanfic and screenies of items you like the look of). But if you can get the book at a good price, it's worth the read.