Granted I'm on a crop sensor (60D), but don't overlook the wide end of those zooms. I've got the 70-300L, and 70 is a pretty useful for landscape and portraits.
I do a lot of bird photography with this lens as well. I've never used the 100-400, but my guess is that in many situations you won't do any worse with the 70-300L. You're going to be cropping with both lenses (because, you know, birds are jerks), and the extra sharpness and better IS of the 70-300L will at least make up for the extra reach of the 100-400L.
Here's a recent set.http://www.flickr.com/photos/12341100@N06/sets/72157632681526309/
I had the 70-300 IS USM, and yeah, the 70-300L is definitely a great upgrade from the nonL lens.