I use a 10d and the 100-400L IS. I have no problems with dust. I'm
fairly careful changing lenses but it's nothing that any good
photographer doesn't do with any camera and lens. It's just not an
issue for me.

Yep, that it is. Here's mine. I haven't used a 300/4 IS, but I heard
it was less sharp than it's brother the 300/2.8 IS.

I'm very pleased with the sharpness of my 100-400L IS. I use a 10d and
zooming in to 100% on a Heron's eye, it appears damned sharp. Going
over 8 by 10 is no problem at all. But then again, I don't view them
from the same distance as a 4 by 6. Large prints are to be put on a
wall and viewed from some distance, not examined with a loupe. And
let's be honest, the emotional feelings from an image are still the
most important thing to me. Technical aspects fall to the wayside to a
good image. If it looks sharp, it's sharp enough. I tire quickly of
these nitpicking discussions about sharpness. For some, the 100-400
isn't sharp enough, but for me, it is.

The 300/4 IS is a good option granted, but it requires adding or
removing a teleconverter, and there are different tradeoffs. My 1.4x
lives on my 100-400L. If I had the 300, a 1.4x would also live on it,
but it wouldn't be as long. Both would lose sharpness, but hey birds
don't let you get that close :-)

No, a longer lens like a 500 or 600 would not suit me. I can't shoot
my hand holding walk around style with a 500 or 600 lens. I like
compactness and carry ability. I don't like standing by a large pod
waiting. If you shoot mostly at places where you can't walk around but
everyone sets up a pod and long lens, then perhaps a 600 would be
best. I find my nature in natural surroundings. And often far from
where the car is.

It's well known that you never really have too long a reach with a
bird lens. I am often still cropping images. But then my aim is to
produce art, not technical masterpieces. Art that is viewed from
proper viewing distance I might add.

I'd love to have a longer zoom that was the same size as the 100-400,
but that ain't going to happen, at least until Canon gets busy with
it's DO technology. I'll be in line with cash in hand for the new
200-600L IS DO, if it fits in my Lowepro TLZ Pro AW.

If you add the 1.4x to the 300, you end up with less sharpness  than
the 100-400 with almost the same FL. If you add a 2x you get less
sharpness than the 100-400 with 1.4x and about the same FL. I will
agree that you get a slightly sharper image at 300mm but I still
maintain the difference on a large print viewed at normal distance
does not exist. The 100-400L IS is very sharp at 300. It's just that
hardly anyone has ever used it at 300 :-)

Oh I should mention that I can AF with the 100-400 and 1.4x with the
help from the 'bit of tape on a contact' trick. Really, it works very
well for me.

Just my opinion of course.