I still, after much usage and thought, give the TOP CHOICE to the Sigma 18-200 OS for its TOTAL CONVENIENCE with similar imaging quality (even their MTF charts are similar). I don't find a meaningful difference in AF speed between them, and while the 18-200 OS has a little more AF noise, it isn't objectionable. The 18-200 OS gives you an extra 75mm at the long end, and is also THE FASTER lens when you look at their focal length to max aperture breakdown. Both are significantly faster than the 1785 IS.