It seems like your decision isn't going to be made by differences in image quality between the new 500mm and 600mm, though it does look like the 500mm does hold a slight edge based on the MTF chart (and I thought the new 300mm f/2.8 had an awesome MTF chart... ok, it still does, can't wait til I get mine).


I think you need to look at some of the shots in your photo library taken with the 500mm and ask yourself if 600mm would have been too long or if you would have preferred to have more reach?


It looks like the 1.4x and 2x III will be very usable on either lens. I just got it them both last week and have yet to take anything other than test shots with them and my 70-200mm f/2.8 IS II. I was impressed with the test shots. The 1.4x was awesome, even wide open at f/4. The 2x was very nice at f/5.6, but of course stopping down to f/8 helped. I never owned the 2x II so I have no comparison, but I'm happy with my purchase. I mostly bought them with the thinking that they'll eventually be paired up with a 300mm f/2.8 II.


How was traveling with the 500mm? You mentioned going to Yellowstone with the lens. Part of the reason I was looking at the 300mm f/2.8 is that I thought paired with a 7D and maybe a 1.4x (300mm * 1.4 * 1.6 = 672mm) it would be perfect for traveling and hiking, grabbing wildlife shots when I got a chance while 300mm might actually still be useful for some far-off landscapes.