The Canon rated ever so slightly better optically over at Dxo's website tests. I doubt that you would be able to discern any difference in field tests...Here is their rating & then a link to the site...Note that the Sigma rated higher than the Canon non-is model you have...
1st place at 28: Carl Zeiss Makro-Planar T 100mm f/2 ZE Canon.
2nd place at 25: Canon EF 100mm f/2.8L Macro IS USM.
3rd place at 24: Sigma 105mm F2.8 EX DG OS HSM Canon.
4rd place at 23 (tie): Tamron SP AF 90mm F/2.8 Di MACRO Canon and Sigma 105mm F2.8 EX DG Macro Canon.
pigpen wrote:
I own the Canon (non IS version), and I love it. It is extremely sharp. Regardless of how good the Sigma may be, I doubt it would be as sharp as the Canon. I'm not knocking the Sigma, just sayin' the Canon is so good, I doubt any lens could compare.