After spending a few hours of research, I have come up with two main differences between these lenses aside from price of course! First, Sigma has no IS ability. Second, Sigma lenses are generally less sharp that Canon counterparts. I'm just curious to know your opinions. I'm leaning heavily toward Canon, but my wallet is leaning a different direction.
The Sigma is also 40% heavier (hence the name Bigma!). I choose the Canon because it has IS. I can handhold the Canon down to 1/125sec, and I doubt I could do that with the Sigma.
Here is a thread that discusses the Telelphoto zoom choices. One other option you have is the Sigma 80-400 with OS - Sigma's version of IS.
I ended up with the Canon, but am not yet worthy of it's abilities. I did steal a little time from the guys at Wolf and liked the Canon better while in the store. The Bigma was stiff, and I could definitely zoom faster with the Canon. I guess it's all in what you get used to.
I have no experience with the Sigma 50-500 but I have tried the Canon 100-400 L IS and the Sigma 80-400 OS. The resolution of the latter is significantly higher. I have posted some crops for comparison here :
I can't say anything for the Canon, since I am not an owner of one. But, as you, I had the same dillema. I deceided to go with the Sigma mainly do to the price. It's a top notch lens. Both these pictures were taken with it.
Unfortunatelly, it is very heavy and I seldom use it, oh, and it's not white, but it produces some awesome pictures!
The second picture is very blury as you can tell. I was on the Lake Michigan coast and the winds must have been over 30MPH! I couldn't zoom any closer since the images would be unusable. I didn't want to push the ISO on my old 10D because of the noise. Just imagine the pictures at 500mm. I'll look for some on my hard drive.
