I have the Canon 100-400 L II and it is by far the nicest lens in its class I've owned and never regret buying it. I had to read the Ken Rockwell review to learn about the Tamron. The Tamron saves you a lot of money, and surely would be a good tool, but it's really not in the same league for a whole bunch of reasons. The biggest difference I see is that the Canon lens focuses at three feet and essentially doubles as a macro lens for small moving critters. If you have to live with one for years then it's an easy choice.
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