choster - 4:16 pm on Mar 27, 2012 (gmt 0)
The answer is neither; you want a responsive design. For example, a traditional 3-column site would have three columns of a certain size for fixed width, and three columns of varying sizes for fluid width, but the truth of the matter is that on a phone, you may have only space for one long vertical column in portrait and two columns in landscape, whereas on a 16:9 desktop monitor, four or five columns may make sense.
Just as no one maintains separate text-only websites, or IE- and Netscape-specific websites as we did in the early days of the web, having a separate mobile version of the size is increasingly a losing proposition. You have phones with 3-inch to 5-inch screens, 7-inch and 10-inch tablets, and all manner of desktop sizes, and unlike the desktop world, people are increasingly accessing your site from a multitude of devices and displays. Better to learn a responsive framework like Less, Golden Grid, Skeleton, or what have you now and build for the future.