Illustrator would be way better.
Disagree...Illustrator is great for generating a shape using bezier curves, but for getting it scaled properly to actually build a guitar off of, nothing could be easier than using a raster image. 300 = 1". Piece of cake. Now he could scan the drawing, import to illustrator, and then create the bezier curves over the top of it to give it a good clean outline, then import to photoshop for scaling. That's what I would do. But in my post I was only referring to the 'scaling' aspect of things, not the 'design' aspect.
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I must still disagree, for scaling vector-based wins hands down. The beauty of vector-based graphics is that you can scale them all you want without loosing quality, with photoshop you'll end up with a blurry image.