Looking into getting one of these lenses used (keeping the Sigma tele!) on ebay. The 28-70 goes for about $200 cheaper. I plan to use it for my high school paper (I absolutely NEED a 2.8; my school is pretty dark and as immoral as it is, I have become a 2.8 snob...:blushing, senior portraits, high-quality vacation photos, and just because I want a bit of a boost in the quality of my photos that I can get from a nicer lens. How do THESE lenses compare in terms of quality (contrast, color, sharpness, etc)? I don't consider just the extra 4mm on the wide end to be worth the extra money, but a big difference in something else, maybe.... Expert opinions, 2 cents, something in between? Thanks!