Quote Originally Posted by neuroanatomist View Post
The problem you'll run into with the 400/5.6 is without IS and on a crop body,
The IS on the 300/4 is only 2-stops, but with a 1.4x TC that means you can be down around 1/200 s and still get sharp shots when handholding (of course, that won't work if your subject is moving with any alacrity).
I would go with the Canon EF 100-400mm f/4.5-5.6 L IS USM, It is sharper than the 300 + 1.4 and has IS. A few more buck$ but not much.