Ben,
I started out this week planning to get another 200mm lens. I have 2 Canon DLSRs that I use them to do sky surveys (for comets, nova, etc) and use the 100 and 200 lens. The 100 lens was starting to get too small for what I needed. I was even looking at that lens you offered
Then came the 300 2.8 and couldn't pass the offer up even if it is a non-DG.
I can tell you the Sigma 300 lenses are superbly sharp. Even my non-DG version is almost equally sharp at f2.8 as it is at f8, the problem being the flaring effect. In daylight shots this results in loss of contrast at f2.8 (making the image appear soft), so you need to stop to f3.5 to get punchy sharp images. I have corresponded with somebody with the DG version and he says it is as sharp at f2.8 as it is at f8 (and is superbly sharp at that). I think you can pick up one for under $4K in Aus but not sure if the Missus wouldn't find out and you'll have trouble hidding it! Also its a lot of lens to lug around why not go for the Canon 300 f4 IS?
Terry