Why is the W3C box model considered better?

One word answer - -box-sizing

You choose how you want your box model to work.

Not everyone considers it to be better. To extract a quote from Quirksmode.

Personally I find W3C's box model counter-intuitive. To quote myself:

Logically, a box is measured from border to border. Take a physical box, any box. Put something in it that is distinctly smaller than the box. Ask anyone to measure the width of the box. He'll measure the distance between the sides of the box (the 'borders'). No one will think of measuring the content of the box.

Web designers who create boxes for holding content care about the visible width of the box, about the distance from border to border. The borders, and not the content, are the visual cues for the user of the site. Nobody is interested in the width of the content.

I agree, the border-box model makes more sense (at least, it does to me). There were disputes over the original W3C box model, leading to the definition of the box-sizing property in CSS3.

Personally, I prefer -to my occasional shame- IE's box-model. As noted by the OP it seems to make more sense to have a pre-defined width from which margin, padding and border-width are subtracted, than to have a width to which these are then added.

On the other hand, I can work with both models quite happily, all I really want is consistency between implementations, whichever implementation is chosen.