Or go for the big iron, the Sigma 300-800 zoom. You might want as much reach as possible. Otherwise, Michael's review is an excellent statement of the facts. And my preference would be for the 500mm based upon size.
Even with a bird feeder about 10 meters from our porch, a Canon 100-400 on a 30D, 1.6x, isn't enough to fill the frame with smaller birds like chickadees. So sometimes I will use a 1.4x Tamron SP or 2x Canon TC. And rue these cloudy Northeast winters were even at ISO 800, I am shooting at 1/60 sec at f 11. At least with digital I am not paying 50 cents per image as film would cost me.