A bare 100-400L is a great lens. It is versatile, and has IS. It is not going to be great in low light, but you are going to pay much much much more for that privilege. I was faced with the same choice as you a few years back, and I chose the 100-400L over the 400mm. I have not regretted it. The IQ is great if used correctly. Here are a few examples with a 7D (same sensor as yours)... if you want to see a larger size, then click on the picture or title and you can see larger sizes in flickr:


Green Heron-7893 by westmichigan, on Flickr


Duckling-3911 by westmichigan, on Flickr


Twin Fawns -1000 by westmichigan, on Flickr


Deer Along the River-9866 by westmichigan, on Flickr