Klaus,
After two years of using the Canon 100-400mm, I can only recommend it! I have taken about 20,000 photos with it, hand-held, and out of those maybe 50 photos have been crap, meaning out of focus. I had the Sigma 150-500 before and it was useless, I had hundreds of photos of ships with sharp bows and unsharp superstructures! So I can only recommend spending the extra money to buy the Canon 100-400mm, it's truly an amazing lens and worth the extra money!
Cheers,
Aleksi