For sports shooting, that's a bit different than shooting from a boat or landscape photography, like I was using it for. For sports, you'd probably benefit from the fastest glass you can afford. The only problem there is that you'd have to dump a disproportionately large amount of cash into a faster lens. For example, I think the 70-200 IS f/4 is around $1000 and the f/2.8 version is over $1500, and neither of those goes up to 300mm. Clearly you just have to decide what is/isn't worth your money for the types of shooting you want to do. For me, I considered the Canon 70-200 IS f/4, but that one had divorce papers written all over it, due to the price.
For macro work, if you use a tripod, it probably doesn't make that much of a difference if you have IS or not (assuming your tripod is good). I think I'd get the Sigma if that was what I wanted to use it for, and not waste money on the IS or fast glass. If I didn't want to use a tripod or didn't have a decent one (I actually don't have a decent one, at least not now when Christmas is a few months away, still) I'd prefer the IS, since its real tough to hold a 200mm or 300mm macro lens steady with just your hands.
So... If I shot sports I'd have decided differently than the Canon with IS. If I wanted it primarily for macro and had a decent tripod, I'd have decided differently, too. I don't think you have to have the really expensive lenses to take good or great photos, so if you want to save a buck on the Sigma, I don't have anything negative to say. Just make sure you can live without the IS.
Also, keep in mind that Sigma and Tamron are now making lenses with their version of IS, and the newer Tamron ones have MACRO functions.
This is just MHO.
I have already decided on the Sigma for my first lens. But when I talking about the other 70-200mm Canon L Series im talking a whole new purchase. Which wont happen for a while! Lol, and if I do get that lens It will be for shooting sports only. For Christmas I think ill get the Canon 100mm Macro lens, since Macro is the type of photography I'm most interested in.
Thanks everyone!