this is with the canon on a 300D
i would have thought that 70-300 was a perfect compliment to 17-70 but each to his own
i wouldnt put too much store in the macro capability of the sigma as none of the jack of all zooms offer true macro , imo you would be better off with a 50mm with extension tunes from a macro point of view
there is nothing inherently wrong with the sigma except the noisy focussing but imo the the canon is better.