Have just spent the last 12 months making the same decision.
First a confession, I’m not a good photographer, let alone a good bird photographer.
I went for a Canon “L” because I wanted the best IQ and IMO none of the third party lenses come close:
100-400L
Cons
Old design (1998)
Pros
Is getting cheaper all the time, Canon U.S. took another $100 of the RRP this year.
2-Stop IS
Works with all the standard TC’s, but AF may not on your body
Zoom Lens, greater flexibility
1.380Kg
400L f5.6
Cons
Oldest design (1993)
No IS
Fix focal length
Pros
1.250Kg
Is getting cheaper all the time.
Works with all the standard TC’s, but AF may not on your body
70-300L
Cons
Doesn’t work with standard TC’s
100mm less than either of the alternative.
Pros
New design.
Best IQ @ 300mm - See link below
Zoom Lens, greater flexibility
4-Stop IS
1.050Kg
IQ comparsioin between 400L and 70-300L at full length. IMO the 70-300 is sharper but the 400L has less CA.
http://www.the-digital-picture.com/R...mp=0&APIComp=0
In the end I chose the 70-300L because its new technology which should allow it to keep its price. Its lighter, at just over 1Kg its going to be easier to work with. I need both IS and AF, as my eyesight is not the best.
The only problem with this lens is that missing 100mm.
Hope this helps
PS: For those who love the 150-500, there is a Crop IQ comparison chart at 300mm to be fair, at 500mm its very bad:
http://www.the-digital-picture.com/R...mp=4&APIComp=1