I had Sigma and it's wonderful lens. Then I had one non-is Canon, and now I'm waiting for another one... non-is of course. There's one difference only, and it's not faster af, better saturation or something... It's just from what body of lens is made. Sigma has a bit more plastic then Canon. And NO IT'S NOT PLASTIC!! It just has few more parts made of plastic then Canon does 
That's it. If you will see difference in af speed then you are good. I didn't see it on any of my cameras (1dmk2, 1d, 1v, 3). I went with Canon because I do shoot a bit more then average user, and in a bit worse conditions then average user... and Canon has one nice thing called CPS 
As your comment about 300/2.8 lens... if I do suggest giving chance to Sigma 70-200/2.8 I don't think I would suggest anyone to get any other (Canon compatible of course) 300/2.8 lens then Canon. For a bit more you get real beast. If anyone thinks 70-200/2.8 is sharp then they didn't even see sharp lens yet. So if 70-200/2.8 might still be in reach of hobby photographer, 300/2.8 is lens for working pro. You don't buy lens like this to take photos of your kids in backyard. You spend that money, and helluva lot of it, for tool to earn money. And with that... well... it would be your decision anyway
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