Back when I was young a naive and just starting out this wonderful world of computers, my default stance was, when in doubt buy the thicker of the two books since it must contain more information. I distinctly recall standing in a book store and working out which massive Linux book would give most pages pr dollar.

Now my default stance is, when in doubt, buy the book that's under 300 pages, since if you can't cover you're points in less than 300 pages you're either trying to say way too much or your book is mostly fluff and filler.

Also I imagine the ubiquity of fast internet connections and Google has made the 1200 page "complete reference to everything" type of books more or less obsolete. More and more books these days are "focused expert knowledge in one particular area" which is a lot more suited for the 150-250 page format.