Well, one reason might be cost. The Ciao is cheaper than the Sketch. You cannot get as many colors in the Ciao (144) as you can a Sketch (322 colors). Also, you can airbrush with a Sketch (you have to get the airbrush thingy to do it), but I would swear I read about someone rigging it to work with a Ciao somewhere on SCS...
Also, they are a different shape. The Ciao is the same shape as a SU marker, whereas the Sketch is more of a wider pen. I am not talking about the tips, but the actual pen itself.
I went for the Ciao because, lets face it, I am no artist! I do not really need 322 colors or the ability to airbrush! I wish I were that talented...
HTH some..
You can always go to their website at copicmarker.com for more info.