What Julius said.
But yes, the 5D does track better than the 20D, in my experience.
In Tibet, my friend and I shot flying ravens in the afternoon (around sunset, but there was still sunlight). If I recall correctly, I used the 70-200 f/2.8 IS, and my friend the 100-400 IS. I got one or two usable, in-focus pictures, he got at least five, and I think I took several more.
It's possible that my technique was at fault, but I also got better focus tracking with the 5D when I tried it for myself.