Thanks rawshorty. Yea I think the Sigma is the cheapest option to get more than 400mm.
Is it really bulky and heavy for handholding? How's the sharpest wide open @ 500mm?
The Sigma is not unwieldy to handhold. But it is long for storage. The 100-400L would fit in any of the bags I use. The 150-500 Sigma will not. It is ok wide open, but better stopped down.
I've thought about going to a 100-400, just because it would be easier to take with me, because of size. I find the IQ and performance of my 150-500 to be at least near enough (almost the same) to the 100-400L that that is not the issue for me. I just can't give up the extra 100mm.
I want the zoom for flexibility. I could see picking up a 400mm f/5.6 L for the AF speed and it does have better IQ (IMO) than either zoom.
If I had the 100-400, I would not switch it out for another zoom unless you like buying and selling AND really must have the extra 100mm in a zoom. From what I here the 70-200L IS MKII and a 2X (III) is close enough to the zooms for many that occasionally use the combination. (I have the 70-200L IS MK and it is not an option.) I doubt either zoom + TC is what many would settle for if they used the combination often. You sound like you will be using it regularly not sporadically.
I think you need to decide if you want the flexibility of a dedicated long zoom or the better IQ of the 400mm prime.
That said, I think that for you the 400L (f5.6) would probably be your best bet (mainly due to IQ). Use your zoom with a 2X (I'd rather use a 1.4X) when not using the 400L. If after you have used that combination enough and still think you have need of a longer zoom, then pick up the zoom that you think fits your shooting the best.











