At first, I leaned toward a Canon 100-400 L lens (I already have a Canon 400mm prime, a fantastic lens, as long as the subject is out there a ways from you) and then I saw the Canon 70-300 L.
I read that the IQ and AF is better/faster on the 70-300 L, than on the 100-400 L
I know a few birder friends that post pics on the daily bird pics thread and I've seen that the 100-400 is certainly no slouch, the pics they post are tack sharp. But, I don't recall seeing any pics taken with the 70-300 L, nor do I know anyone who has one. I don't know if I'll like the push/pull thing the 100-400 does.
Do any of you have both? Any thoughts would be appreciated.