I am looking to pick up a zoom in this range and I can't afford (wife won't let me spend the money) for the Nikon 17-55 2.8. What do you guys think of this lens? I am between this one: http://www.adorama.com/SG1770HNKAF.html and the Tamron 17-50 2.8. Here is the link to it:http://www.adorama.com/TM1750NKAF.html I have heard great things about the Tamron but the extra distance with the Sigma would be nice as long as it is as good of a lens. What's everyone's opinion on this?