I have gone with 500mm twice. (both IS versions, 12 years apart)
Three reasons for me;
- Weight/size
- Field Of View for birds in flight. (if the lens is too long, the field of view is so narrow that acquiring a bird in flight becomes much harder. I found 500mm on APS-H to be about the limit to how long I wanted for that kind of work.
- Price
However, Two of those reasons are easily dated in today's world!
- The Gen III 600mm weighs LESS than the current Gen II 500mm! Now the lightest option is the 600mm! Crazy.
- I was shooting on crop sensors (1D APS-H 1.3x crop) In years past, the only sensible bodies to mount these one were crop sensor bodies. This has changed. On a full frame sensor, the 600mm FOV is wider than a 500mm on a crop. Now I shoot with 5D4, R5, or 1DX, all are full frame. Now to get the same FOV I am used to I need to ADD a 1.4x t-con to the 500mm.
In a perfect world I'd get the gen III 600mm today, if I felt I could afford it.
Of course, if you aren't in the market for a Gen III 600mm due to cost, then the 500mm II retains the weight advantage.