I own the 100x400 and (if the light is right) its a very good lens though can be a tad heavy if walking the forests etc.
However Canon have just released the 70x300 L (see link) and I plan to rent one in the spring to see how it fares
Whats worth thinking about is the extra 100mm (with your 1:6 crop ratio) on the 100x400 will give you far better telescopic access to small birds over the 300mm
http://www.usa.canon.com/dlc/control...articleID=3512