Image from Henri Rousseau's, The Sleeping Gypsy.
I have two children - a daughter and a son. Separately, each one will ask who is my favorite child. My response is always, "I don't have a favorite." My daughter might be better at one thing, my son at another. It is the same answer that I give when asked what my favorite meal is - What are you in the mood for, Italian, French, Chinese, ...? Diversity is a great thing. I like choices. Choices make the world a more interesting place in which to live.
Choosing a layout is a confusing mix of facts and questions. What type of content will fill my site? What is the purpose of my site? Who will read the site? And on and on. Surely, a web designer would agree that the layout for the N.Y. Times and the Huffington Post will differ from Amazon or Overstock. Or that a photographer's site will differ from a butcher, baker, or candlestick makers. Then, there are bloggers, but they too come in all shapes and sizes. (Edit this paragraph.)
The 960px layout provides a great amount of territory in which to place content. For newspapers and sites in which content is more important than design it is the ideal layout.
However, a 780px or 800px layout may provide design layout advantages. The primary design advantage is the use of white space to highlight the content. At least one graphic designer says,
At least 20 % of an ad should be blank (white space). Ample white space helps gain attention, create contrast, and unify the ad. According to Albert Books, white space is probably the most underestimated element in advertisements.http://www.wiu.edu/art/courses/handouts/layout.html