1) The 70-300mm has a newer, 3-stop IS system, compared to the 2-stops of the 100-400mm.


2) The 70-300mm is lighter.


3) At maximum zoom, the 100-400mm has a 33% longer focal length.


All of that means the IS on the 70-300mm will be relatively more effective than on the 100-400mm, especially at maximum zoom. But, you can certainly have the 100-400 checked out, just in case...