People often say that they choose the f/4 over the f/2.8 as they rarely shoot wider than f/4. What they forget is that if shooting at f/4 with the f/4, your shooting wide open. If shooting at f/4 with the f/2.8, your shooting I stop down from wide open.
Everyone egrees that the Canon 70-200 range is a good set of lenses, but most would also agree that a lens performs slightly better when stopped down slightly. The other advantage is that the f/2.8 will give a slightly brighter viewfinder.
Ultimately the choice is down to the individual, and in this case you need to weigh up which matters most to you, IS or the extra stop.
p.s. Some great images there Cuthbert.