« on: February 28, 2012, 07:24:25 AM »
I have not shot the 70-300, and so I cannot comment on it. The one drawback that I see to this lens is the speed. With any indoor activities or sports in less than bright conditions, it will be a challenge to get the shutter speed you will need to stop the action.
I have the 70-200 f/4L IS for one year. It is very sharp, colors are vibrant, contrast is excellent, the IS works well, it focuses quickly in good light, and it is fairly light in weight. It also has weather sealing, although I have not put this feature to the test. The drawbacks are that it hunts a little in low light and its speed. Like the 70-300, the speed will be the key weakness in using the lens. The lens itself is amazing. In decent light, my keeper rate is high.
Another option you may want to consider is the 70-200 f/2.8L II. It's comparatively expensive, and it is larger and heavier than the 70-200 f/4L IS. The extra stop of speed will make a big difference if you are shooting in low light and/or indoors. Also, if you get an extender, you will get almost the same reach as the 70-300. I have this lens and the IQ is roughly the same as the 70-200 f/4L IS. The big difference is the speed of the lens. I often need the speed more than portability, and therefore it is a better match to my shooting than the 70-200 f/4L IS. Regardless, both are great.