70-200mm 2.8 IS with a 1.4 extender. It is the better one for sports (f2.8 nice and fast), portraits (2.8 gives really good bokeh), weddings (low light).


The 100-400mm IS is more for nature and aircraft shows are another favorite for it.


I had that same problem when upgrading the 70-200mm f4 L until I spoke with someone who sold the 100-400mm. They said it can pick up dust due to the push pull zoom acting like a bellow. So I went for the 70-200mm f2.8 L IS for all the things I listed as its bonus points as well as the dust issue.