100-400mm canon opinions

I have a 7D and 70-200mm IS II lens.  Recently I have started taking some pictures of birds on my feeder right outside my back door.  While some turn out OK...I lose a lot of sharpness when zooming in on the picture after its taken.  It is very apparent that the 70-200 is not going to fit the bill, even with the extender on it.  I am thinking about getting the Canon EF 100-400mm f/4.5-5.6L IS, and wanted to get a few opinions before dropping the cash.  I know that Sigma also offers the 150-500mm f/5-6.3 for slightly less.  I have until the end of the day to take advantage of the $200.00 off the price of the Canon lens so I have about 14 hours to make a decision.