|Fully zoomed, the 70-300 is at F5.6 wide open. I don't know about at 200mm.
Fully zoomed, the 70-200 is at F2.8 wide open.
Neither camera produces best results at these apertures, but in light-gathering ability, the 70-200 knocks the 70-300 about. These 2-3 stops difference in aperture pretty much negate any difference gained by IS, but you will get 2-3 stops worth of shutter speed if needed. That's great because it can help you get better looking pictures if the motion is not in your hands.
Photographically, the 70-200 is better.
Having an extra 100mm is ok, but most people say that for really high end work, the last 70-100mm of the 70-300 is soft. Only you can say if it is within your personal tolerance for softness/sharpness.
On the other hand, what bear says about physical size makes a difference too. I just had a bit of a surprise that the lens my friend wanted to sell me isn't the 70-200 f2.8, but is the 80-200 F2.8. Having given it a bit of thought, I'm pretty glad about the misunderstanding. The 80-200 is much closer in size to the 70-300 and has Internal focusing.
It might be worth a look for that lens if you want a bit of a comprimise in size and performance. It is old and discontinued, but I hear it's still sharp enough to keep head and shoulders above the 70-300. Check fredmiranda.com and local shops for second hand deals!
Alternatively, seeing as price is not an issue, you could also consider using the 1.4x Teleconverter. This will bring your telephoto end up to 280mm (in the glass) and your max aperture will go to (I think it's) 4.0. That's still pretty decent and what's more, it still constant aperture, so as you zoom in, your focus stays put. That can be an issue for some. For others, it's completely unimportant.
Message edited by author 2006-01-11 22:40:39.