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I'm going to have to pitch in for the 15". I know I supported the 12" earlier, and I still support it, but I concede that that a 15" will be much better for graphics work. I was working in Flash today on my 12" and it was a pretty big pain. However, I also agree that the Powerbook may not have the best screen if color is critical. When I hook up to my 17" Samsung, the differences in contrast and saturation are dramatic.

So I'd say 15" if you need work on the go. But a 12" + LCD combo would make for a great mobile all-around computer and graphics workstation setup.
 
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