Best Barrel Length?

[Whelenon]

I'm told the optimum barrel length for the Grendel cartridge is 19.5". True or false? I have on order a 16" barrel(mid-length) and an 18" barrel(carbine-length). Which one to use? The rifle will be used for deer hunting, 150 yards and coyotes, 300-400 yards. Trying to keep the weight to a minimum but accuracy trumps weight. The 16" will probably arrive first, do I use the 16" or wait for the 18?

Whelen