The 90-300 lens would be the best buy as it has a USM drive which is quicker and quieter to use. The other thing I'm not sure if you know is as these are not dedicated digital lenses the lens focal length will be 144-480 when used on the 400D as it will be magnified by 1.6 . I have a 400D myself and find no trouble at all when I use mine. Peace , Lubin