Tony, I tried the Sigma 18-125 on a Pentax 1stds and I was unimpressed (actually crap was the first thought that came to mind). The chromatic abberation was extreme to the extent I decided not to go with the Pentax Sigma bundle. I took a few shots of some buildings close by and the edges on one side of the image were fringed vertically with purple, on the other side green. That said the performance of that lens may not have been typical. I'd go for the 17-85 IS Canon Lens it's image stabilised and will be a much better lens. Don't know much about the 20DA but I believe it's only for astro use and you may have to add an external filter to correct for daylight photography. Try this link for some info on Lenses for EOS series.
http://www.bobatkins.com/photography...00dlenses.html