It depends on your network latency and for images of non-trivial size, with bandwidth available both on your server and on the user side. High latency and low bandwidth will increase the likelihood that the canvas based solution will be faster.
The speed of drawing an image on a canvas also varies greatly between browsers. Here are some pretty outdated results:
http://ajaxian.com/archives/canvas-benchmark
Since then, Chrome has released 4 versions, the Firefox team has worked on Firefox 4, and IE9 with hardware acceleration. But that can give you an idea of ββvolatility, at least.
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