From another thread...
maxjj wrote:
Hi Popo, cannot remember if I have asked this before or you have answered before, but how do find the 100-400L vs the 70-300L.
I love the size and portability of the 70-300L, but sometimes want more reach.
Up front, I'd say if I could only have one, it will be the 100-400L without question. I could repeat the stats and their differences you see in all the reviews but wont. There's little details you don't really see in a review which may swing it.
Something you may not see in the average review is the 70-300L isn't parfocal. That is, as you zoom, the focus changes. The 100-400L doesn't noticeably suffer from this, and I think this helps contribute to AF tracking if you're zooming at the same time. The 100-400L also resists focal length shrinkage as you focus closer, which the 70-300L does much more noticeably.
Handling wise, the push-pull is just a lot faster and more accurate, all the more so since I don't like the reversed ring layout and would prefer the zoom to be closer to the body for better balance. The weaker IS can be a disadvantage if you're going for a longer exposure, but generally for wildlife you need to keep it somewhat fast anyway to avoid motion blur. Image quality, if you pixel peep you might be able to find differences, but to me they're both equally usable wide open.
Of course, I still got a 70-300L. It has better weather sealing, and is smaller and lighter although to me the difference there isn't significant over the 100-400L in 99% of situations.