3D - I believe, but I am not 100% sure, that the 100-400IS is a better lens, optically, than the 75-300IS. I have never used the 75-300 so I'm not going to try to compare, but I have used the 100-400IS. It's a great lens, but it is also a heavy lens. Keep that in mind. I would ask you to take a look at 3 Canon zoom lenses, same focal length with different technologies, and consider them: 1) Canon 70-200mm f/4.0L - ~$600 2) Canon 70-200mm f/2.8L - ~$1100 3) Canon 70-200mm f/2.8L IS - ~$1600 All three are fantastic lenses. The 70-200 f2/8L IS lens is phenomonal. Great sharpness, clarity, contrast and the IS gives you 2-4 more stops of shutter speed. So, for instance, conventional wisdom says that if you were shooting at 200mm then your shutter speed needs to be at least 1/200th sec. With the IS on, that minimum shutter speed becomes 1/50th or even as low as 1/10th of a sec. You also need to take into consideration that the 70-200 becomes a 112-320mm lens on the 300D. Just a thought. Another lens I liked a lot was the Canon 35-350mm f/3.5-5.6L L zoom. Had a nice 2ft close-focus distance on that one. Would pick the 70-200 2.8L IS over it any day, but it's a nice lens too. -=>Donald