Well, the
Sigma 100/2.8 Macro is around $480 and the Canon 100/2.8 Macro USM is around $490 from reputable dealers if you can find one in stock anywhere. Right now they are backordered at both
B&H and
Adorama and the dealers on Amazon are marking up the price.
The Sigma is certainly no better, doesn't have a ring USM motor and probably doesn't have the build quality of the Canon, so I really see no reason to go for the Sigma, other than the fact that you can probably easily find one in stock while the Canon lens seems to be in short supply at the moment.
If you want to save some money you could look at the
Tokina 100/2.8 Macro is around $400 and gets good reviews.