I have tried both for some time now, fixed width and fluid width…
Each one has their advantages and disadvantages, but from a pixel-perfect take fixed width is the best because what I see on the Sparkle canvas is exactly what I get when viewed on the device! 
Fluid width on the big monitors for me is a mess as everything is stretched out to fill the space which in turn rearranges the layout. The stretched out text boxes becomes a problem for efficient reading… Keeping within the 50-75 character range per line will give your visitors a far better and attentive reading experience, as opposed to 90-140 characters per line.
I totally prefer fixed width over fluid width any time of the day!