Alright, my 70-200 f4 just isn't cutting it. With the lovely English weather I need a lens with IS and weatherproofing for those dull & rainy days. The fact that I shoot sports in these conditions every once in a while definitely doesn't help.
Food for thought. The f4IS has the latest 4 stop IS which effectively allows handheld speeds as low as the 3 stop f2.8IS. Thus the only real advantage the f2.8IS has is for speeding up the shutter speed and bokeh. The advantage the f4IS has is being half the weight, two thirds the cost, and the sharpest zoom Canon has made to date.
Which would you pick, and why? If the f2.8IS was as sharp as the f4IS my decision would be made, but it isn't so. I don't usually stress lens decisions, but this is one I'm really screwing over due to both price and similar performance.