18” for 123gr ELD-M should still be 1000yd capable so long as it isn’t a humid day at sea level. It’s a difference of like 40-50fps. Nothing to sneeze at and useful/needed if you are shooting long range. Even at 600yds it makes a big difference. If it is a hunting rifle that maybe a couple of times you’ll take it to a range with 500 or 600yd targets, go 18”. Handier more than the reduced weight.
Are you sure? I thought the higher the humidity the less dense the air. Which means less drop. So higher humidity at any altitude would equate to less drag.