Thanks everyone. I did leave the question intentionally vague as I wanted to see a variety of responses. Perhaps push me to think about this differently. I do agree with HD's point. If the purpose of the trip is photography, then it is more about having the best gear for what you are shooting and you do what you have to in order to get the gear there.
But, for me travel is always about compromising between performance and portability. I should probably say I am very impressed with my Sigma 150-600S. I think the universal consensus has basically moved off of it as a good option, perhaps favoring the 150-600C for a consumer level supertelephoto, the 100-400 II for a more prosumer level, and Canon's big whites for ultimate durability, AF and IQ. But it has really served me well and I am getting a lot of shots I am happy with from it. In general, I disagree with the consensus I've heard (usually from people who have never used it).
While I know that Canon's big whites test as having better IQ, I have actually never had an issue with the IQ from the 150-600S. The issue I know I have is with aperture. To get good IQ, I am almost always (98% at 600 mm) at f/7.1 to f/8. I am almost always increasing ISO (spikes at ISO 640, 1000, 2000 and 3200) and compromising on shutter speed (40% of my shots are 1/500 with another 35% at shutter speeds even slower than 1/500). Looking at those numbers, I am compromising too much shutter speed and I really want 1/1000 or faster.
So, even dropping a stop would be nice and give me additional flexibility on shutter speed and ISO.
Looking over the options, what gets me is the number of good options that exist in this market now. Good, but I would not call any of them "great". Each comes with a compromise, and this is where I get back to most of my shots being taken while traveling. The lenses with the best IQ, AF, etc are even larger and heavier than my 150-600S, which has a well earned reputation for size and weight. Of course, and I am aware of the physics involved, but to get smaller/lighter you are giving up aperture, build quality and, often, AF speed. I have always been generally aware that I use the entire focal length range of the 150-600S, but looking at my 2017 numbers, I am even surprised I am at 600 mm only 60% of the time. The other 40% is pretty evenly distributed over the rest of the focal length range. Put all that together, I would say my perfect lens would be something like the 200-600 f/5.6 that has occasionally been rumored assuming Canon doesn't make too many compromises on IQ/AF, etc. Granted, that is not that different from the 200-400 f/4 TC, which is even bigger and heavier than my 150-600S.
I am going to kick this around a bit more, and I may end up testing a couple of combinations. First, I really do need to see if there is a difference with my 150-600S on the 5DIV. Maybe the improved sensor performance, I'll feel comfortable at higher ISO. After that, I need to consider some options, my current thoughts:
- 300 f/2.8 II (with TCs). One of the lenses that has always tempted me. A bit short for what I want, but f/2.8, amazing IQ, and crazy fast AF.
- 400 f/4 DO II. This may be in my future as it may be an excellent travel lens. This is high on my list for renting.
- 400 f/2.8 II. I know this is thought of more of a sports lens. But as I've shot, sure for birds I am usually focal length limited, but for anything larger I am more often light limited. Size, weight and $$ are the issues with this lens. But f/2.8.
- 500 f/4 II. This is the lens I have always thought I would end up with.
- 200-400 f/4 TC. As mentioned above, if I were to replace the 150-600S, this is likely the logical choice. But, size/weight/$$. But I am very tempted to rent this one as well.
I think the next thing for me to think about is something I know some of you already do. No lens does everything. On important events, perhaps I should have a supertele on one body and something wider on a second body.
This is how I spend my winters.