Firstly to the OP, although I don't have the 120-40p I have the 150-500 which is optically similar. When I was deciding I looked at both lenses and went with the extra length. Check out my Flickr for some examples.
Secondly to Brett the 7D has the ability to micro adjust the focus,as does the 50D and higher end full frame bodies. As far as I know your camera doesn't. However if you send your body and lens to Sigma they will calibrate the lens for you