The problem you'll run into with the 400/5.6 is without IS and on a crop body, unless you're shooting from a tripod you'll likely need 1/800 s to avoid blur due to camera shake, and that probably means a high ISO anyway. THe 400/5.6 is considered a BIF lens, because for that application you need a fast shutter no matter what. 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).