The 70-300L, with it's newer optics and IS system, is a better lens than the 100-400L in every way except for one very, very important way -- reach. As others have said, if you can get by with 300mm than the 70-300L is an excellent lens. However, you mention it will be primarily a wildlife lens. In that case, reach is very important, and the 100-400L is considered a fine lens for this use in this price range.
I own the 100-400L, and would never consider trading it for the 70-300L. I use it almost exclusively for wildlife and zoo trips, though, and have a 70-200 f/4 IS for more general purpose telephoto use. For true wildlife, it usually resides on a 7D, and for the zoo trips I use it in combination with a 5D3. It's focal range, image quality and AF performance on both bodies makes it a really versatile lens for those purposes.