I have used my Canon 100-400 for over Three yrs on sports and I love it.It is a little heavy and the push/pull you will get used to it.With extenders you will lose F stops and AF.I have a 2X extender and I use it for Wildlife.It is a very good Lens. Good Luck with your Choice.
This happened to me, I got a Sigma 100-300 f/4 before I managed to get the Canon 100-400. The Sigma was a very good lens but it just didn't have the reach I wanted or the IS that make's hand held shooting alot easier.
The Canon's now hardly ever off the camera, it's a great lens and worth every penny, if you can afford it... get it!