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Well, comparing depth of field is all well and fine, but the Oly lens still has the light-gathering ability of f/2.8, letting you get a sharp shot in darker conditions. And it is a pro build lens, not consumer plastic, like the Canon.
Carsten you should know better. Sure the Oly lens will have more light gathering ability than a Canon lens shot at f/5.6 on a Canon sensor (at least right now), but that is because of the weakness of the Canon sensor. If it was shot on the new 36 megapixel Sony FF, it would have the same light gathering capabilities because you could just turn up the ISO on the Sony. Light gathering will always be partially a function of the sensor, but when comparing lenses, in my view, it is better to compare how much light they transmit to a potential sensor and because a lens that has a FF image circle projects light across a much larger area a FF lens at f/5.6 projects about the same amount of light onto a FF image circle as a m4/3rds lens projects onto a m4/3rds image circle.
Oh, and note I said the 70-300 f/4.5-5.6L IS, which is not a consumer plastic lens. It is a pro lens as well with about the same build quality as the Oly.