As I have mulled this over, I keep coming back to the 500 f/4 II and 400 f/4 DO II. The 500 f/4 II is the same size as the 150-600S extended (15.3"), and it is half a pound heavier in use weight (7.5 vs 7 lbs). So I expect I would be able to hand hold it just as I do the 150-600S now (as others have mentioned). Plus, it gives me 1 2/3 to 2 stops more light in the range (>400 mm) I am targeting, which is what I want.
The 400 f/4 DO II is smaller, lighter, and less expensive by ~$2,000. Add an extender on it and it will be giving me a 2/3 to 1 stop advantage in the >400 mm range and ~2 stops at 400 mm.
I have the utmost respect for the 400 f/5.6 and 100-400 II. I think they are sharper than my 150-600S at 400 mm and have a 1/3 stop aperture advantage. I would circle back to those lenses if size/weight/$$ became my biggest drivers. And there is something to be said for putting myself in better positions so I do not need 500-600 mm. I consider the 600 mm f/4 to be the ultimate supertele. But I wonder how much I would use it. I like handholding vs shooting from a mount. I take most of my supertele photos while traveling and photography is usually only part of the purpose of my travel. And the 500 f/4 II and 400 DO II are not behind in IQ, just in focal length.
So, I am still in the evaluation process. This is a big enough purchase and I enjoy evaluating, so I have the 400 f/4 DO II coming for my meet up with Busted Knuckles in Florida and the 500 f/4 II for a Presidents Day weekend shoot locally.
BTW, thanks for the tip to pick up lenses at a local FedEx Office. That is what I am going to do.
Thanks everyone....