From: PWW

If you shoot "small" birds, for instance, shorebirds, warberlers and the
like, that extra little bit 600 vs 500 could really help. Arthur Morris
used the 800 5.6, early in his career.

It's timely that you mention Art Morris and warblers since he was just at Point
Pelee NP in Ontario last week photographing the warbler fall-outs and he just
sent out one of his email Bulletins with sample shots to subscribers (I know
several guys on this NG who get this).

Art owns both the 600 f/4 L IS and the 500 f/4 L IS. He made his reputation
with the manual focus 800 Paul mentions and later with a 600 f/4 L but about a
year ago he wrote that he was now using the 500 more than the 600 because of
the excellent image quality and lighter weight.

All the warbler shots in the Bulletin I mentioned were taken with the 500 f/4
and either a 2x or 1.4x t/c on the Mark II body, and they are pretty
spectacular. Anyone using Canon's long telephoto lenses would probably be well
served by subscribing to his free Bulletin emails. I've certainly learned a
great deal from him the past 5 years. www.birdsasart.com

When he posts this Bulletin (# 137) on his website I'll post the link so
non-subscribers can see for themselves.

But a few pluses for the 600;

If most of your competition is using the 500, with the 600 then you have a
little more reach then they do.

And the the images produced with the 600 can be more a little more dramatic
because of Depth Of Field differences.

These are good points ... I wish I had a 600 to go along with the 500 but until
I win the lottery that won't happen And I think for me at least the 500 was
a better choice than the 600 since I have to fly to about 80% of the places
where I use this lens and it's much easier to travel with the 500.

Bill