I'm very happy with my 100-400L (f/4.5-f/5.6 btw), however the 100-400L has about a 2 stop IS and the new 70-300L has the latest 4 stop IS which is worth a lot. The 70-300L will also have the latest coatings for color, contrast and flare which is also a plus.
That said, the extra 100mm of the 100-400L makes all the difference for wildlife. On the other hand, the 70-300L should be just right for sport. Of course you can use both with 1.4 and 2x TCs, but that will mean a 1 series body if you want AF.
...looks like the 70-300L is lighter than the 100-400L, which may be a factor for some folks.
An other preview if you like:
http://www.the-digital-picture.com/R...ns-Review.aspx