300 f4 L vs. 100-400 L?
 

Trying to decide between these two lenses for bird photography (on a digital rebel). Plus maybe on an XL-2 in the future. I realize the prime may be a little sharper and faster, but the zoom a little more versatile and greater magnification.

Any opinions?

i'm in the same cirle as you...and here is how i made my choice

the 300 by all counts is sharper, faster and lighter

the 100-400 is more versitle and has more range...

for me the most important factor is sharpness, then lens weight...

i'm going with the 300, also a bit cheaper

try these links
http://www.fredmiranda.com/reviews/showproduct.php?product=39&sort=7&thecat=2

http://www.fredmiranda.com/reviews/showproduct.php?product=19&sort=7&thecat=27

If it's bird photography you're after, I'd go with the 100-400.
I have, the 400 f/5.6 and I still wish I had more reach. Of course the 300 is significantly sharper, but the 100-400 can produce some very sharp images, plus it's versatile.
They both have the IS if I'm not mistaken.
Have you conscidered the 400 5.6.
The images are very sharp and since I shoot in broad daylight, I don't have any issues withthe speed.
Picked mine up used for $900 and I love it.

Don't forget to get a good monopod with it, and don't waste money putting a head on a monopod.

Steve