All of the input is very much appreciated. Here is what I gather from the replies:
16" - best bet for light, not ideal for the longer end of my 100-500 yard range
18" - gain 30-40 yards on the 16"; middle of the road (kinda like the .40 S&W).
20" - heavier, much better suited for the longer end of my range.
Because this may (hopefully) wear a can in the future I think the 20" is not an option.
I think I have some more research to do. I can say that looking at ballistics tables has been very helpful. Thanks for all of the input!