Macro lens - Canon vs Sigma

I am considering adding a 100/105mm macro lens to my kit in order to be able to shoot shy creatures which are difficult to approach close enough with my Canon 60mm macro lens .... Given the price gap between Canon (Euro 420 VAT excluded) (standard lens not "L" series) and Sigma (Euro 285 VAT excluded) and considering that this will not be my primary lens for macro, I tend to favor the Sigma .... Is there any valid reason I should invest in the Canon lens ???

Thank you very much for your assistance !!!

All the Best ..... Francesco

in my opinion the glass in the canon lens is better, and the auto focus may be a little faster