I have used both the Sigma 50 & Canon 100macro on a 10D.
I like them both very much. I agree that the 100macro is better for macro if that's what you're looking for.
But, in my opinion, the 50 is more versatile. You can shoot both macro and fish portraits with it.
Additionally, you can get much closer with the 50, around 10 inches closer. Sometimes that's quite helpful.
Unless you're really into macro, I'd get the 50-- that is if you're only going to get one of them and if your only other lens is going to be a 15mm fisheye.
I have the Sigma 15mm. I like it very much. I had the Canon for a while and find them to be about the same. I believe most people will say the same thing about the comparison.
Good luck.