I have the 100-400, 300f4 + 1.4TC and the 400 5.6. I gave the 100-400 to my sister and use the 300+ for dragonflies, butterflies and perched birds. I use the 400 for BIF. The 100-400 is one of the sharp ones, it just doesn't focus as fast as the 400. (what does) I got the 400 at Andrew J's suggestion. Thanks Andy!
I have both lenses. 100-400 L is nice to use all day because it is very versatile. 400 5.6L is good for BIF but the MFD is further so you miss close bird shots unless you have two bodies with two lenses.
) I got the 400 at Andrew J's suggestion. Thanks Andy!