How's the af speed on all of these lenses compare? (the Sigma 100-300 and 300 f4L IS with 1.4x tele)
Can you bird with them (100-300, 300 f4L, with teleconverters (1.4x)) or are the native 400mm lenses better?
I think (correct me if I'm wrong), IQ is decent enough on all of them, with or without TC.
I'll be doing some birding probably, mostly other stuff though I think. I ask, because the Sigma is priced best for me, and I like that it's black / has internal focusing (unlike 100-400.) 300 f4L IS is nice, but IQ is comparable to Sigma 100-300, with a 1.4x it apparently still is not up to par with 400 5.6 in terms of af speed and IQ. The 100-400L is nice, but the push pull system might not be for my (tiny hands, short arms, etc), and it is also big and white.
I don't know how much I'll be using the telephoto, but it should be a decent amount of time. The 100-300 does fit the budget best at the moment though.