Well, the canon 100-400 is about 1 lb. lighter, has about a 1/2 stop faster through out the focal range (f/4.5-5.6 as sigma f/5.6-6.3) and is a push/pull zoom but, the sigma 150-500mm is 25% longer in focal length and I believe, ituses a normal zoom ring. The canon has a 1:2.5 magnification ratio as opposed to sigma's 1:5.2. I have the canon 100-400mm and it is very light for BIF photography (light for me as I sometimes handhold my 500mm). I also ocaissionally use it with extension tubes for closeups. The canon has 2 focud limiter selections, not sure if the sigma does the same. Both have image stabilization. Canonhas 2 IS modes. not sure about sigma. Both come with tripod ring.500mm is definitely a plus for wildlife photography, especially smaller birds. The canon 100-400mm is a proven design and the sigma is a new design. that's all i canthink of for now.