One can feel the only point not in a negotiation: The quality is very high in both! I personally find the Sigma as quality. The matte finish EX with the catchy and crisp to rotating focus ring, the right weight ... I just really like the Sigma EX line.
The consistently high sharpness of the optics is quite impressive! I use the 105s for Portraits (unobtrusively, from a comfortable distance), lightweight telephoto shots are (z. B. also together with the C-Kenko AF 1.4x Tele Plus MC4) and of course for all kinds of macros. The results just crazy good , Everytime you plan grabs this lens, it is after the first pictures fascinated by the incredible sharpness, the great bokeh and really very consistent color reproduction. Distortions show up, naturally, in such fixed focal lengths, only minimini minimal and have disappeared with one click in Lightroom.
The 2-stage OS completes its service very good. Along with the good intensity, quite a lot of difficult environments can freehand cope well even where another lens nothing Vernüftiges is already achieved. Just please make dauf - disable tripod at work the OS absolutely! Specifically, the OS evaluates the lens in comparison to its predecessor for portrait and very lightweight telephoto photography. Many macros, however, you will deactivate it (need to).
The HSM works perfectly and quickly, even in low light. Whether the USM of Canon, as alleged here and there, is faster, I leave time to be seen. In the laboratory we discover much that is irrelevant in practical use. When assessing the compliance of the Canon L I for one could not detect any significant speed advantage. Similarly, in the Trefferqoute or accuracy.
When Splash cover the Canon is one step ahead ... if you have the appropriate camera to. I am, however, also z. B. in strong rain, no matter what the objective, never come water into the camera. Not even during the boat ride through the Iguazu Falls. Under such extreme recordings are automatically adapts to that comes not too much water to the camera, because you'll see later, only big drops in the pictures. ;-)
My conclusion: The 50-70 euros for Canon can save you need and invest in a good filter. The Sigma is a 1a-alternative to the original, even if this time the very large price advantage fails.