Choices

I have come to the next big choice of my life. Am I going to spend way too much money and get myself a new lens for my camera, or don't I? And if, which one?
The Canon EF-S 17-85mm F4-5.6 IS USM (on the left) or the Sigma DC 3,5-6,3/18-200 OS C/AF (on the right)?