"Alien Clumps" no to be @ determined wrote

I like the 100-400 in that it has IS, and of course, zoom.

I like the 400 5.6 because it is prime, sharper lense, etc.

So I need some more opinions on the 2 lenses plus some experience. I
sure do appreciate it!

My wife has the 400 f/5.6 L and I have the 100-400 IS and I've used both a fair
amount. I prefer the 400 for shooting birds in flight since it will autofocus
faster, especially with the Mark II. I prefer the 100-400 for shooting from a
confined area like a viewing platform or shuttle bus in Alaska (though I didn't
take it with me on my most recent trip), and the IS is very useful for when you
have to hand-hold.

I also have the 500 f/4, 300 f/4 and 70-200 f/2.8 L lenses so generally take
the 100-400 only when space is a premium and when I think I'll need to
hand-hold a lot. When I drive I usually leave it home.

For most people I think the 100-400 is more useful and a better all-around
choice, and the IS makes it very handy. With the 70-200, 300 and 400 fixed
lenses I'm pretty well covered over this focal range so when space is not an
issue and I won't be doing much handholding I prefer to take two of those three
(almost always the 70-200 f/2.8 and just one of the others).

Get the 100-400 unless you have good coverage at the shorter focal lengths as
well, I feel. The 400 is more of a speciality lens, not as generically useful
as the 4:1 zoom with IS.

Bill