Hi Nikki.
I have the Sigma 50-500mm and although I don't have the Canon 100-400mm lens I do have the Canon 400mm f5.6L lens.
You say you take a lot of in flight shots, the Canon 400 f5,6L lens is regarded as one if not the best in flight lens available. I use it hand held for all my in flight shots and would highly recommend it.
It's lighter than the 100 - 400mm and is also slightly cheaper to buy. Just look in my
Gallery for shots taken with it. Although it lacks the image stabilisation it is light enough to hand hold for in flight and I use it with a monopod for static shots rather than a tripod.
I'm not knocking the 100-400mm it's a great lens as well but I do think the 400mm f5.6L would be more suited to your needs.