Which Telephoto Lens

Hi, I could use some advice. I have had my 20D for several months with the 18 - 85 lens. Love the camera. Am getting ready to buy a telephoto lens for an ubcoming trip to Yellowstone.
I am looking at the Cannon EF100-400 or the Sigma 80-400. There is a $400.00 difference in price.
Looking for advice on which to buy. Any suggestions?

Thanks in advance for your help

The Canon is one of the most popular Zooms Canon makes,. and with very good reason.
The Sigma is nice,. but it is hampered by slow autofocus.

Spend the extra $400,00 now and keep the lens for ever

Same advice here. Spring for the 100-400 if you can afford it. You will not regret it.

My only complaint on the 100-400 is just that it's slow. But, to get a fast 400 requires almost as much money as a 1Ds Mark 2.

Yes. Same here. No regrets.

100-400. If the price difference is now $400, sounds like Sigma's getting ready to "DG" the lens. When I was looking it was only $200. Made it a real no-brainer.