The IS is at least one stop better on the f4 and if you're capturing motion where the IS isn't an advantage it's one level of ISO difference. The bokeh is better on the f2.8. "If you're not sure why you need f2.8 you probably don't" The f2.8 was designed for shooting low light sports, indoors or poorly lit stadiums. It's also a very popular portrait lens. If you're not into those why spend the extra $. One of the major test sites, photozone I believe, has called the 70-200 f4 L IS the best zoom lens ever from an IQ standpoint regardless of mfg, it's that good.
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