AR varmint upper help

[OR_VARMINT_HUNTER]

Looking to build a AR for coyote hunting getting bored with my remington VTR so now want a semi auto question is 18inch or 20inch which is better and why? Also I'm wanting the rock river A4 varmint upper or the coyote rifle upper or the bushmaster predator any one like one more then the other and if so why?
Thanks for all the help

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[Nwcid]

Longer barrel generally = a little more speed.
Shorter barrel generally = a little more handy for moving around.

 

[Twodogs]

My coyote gun is a 6.8spc with a 16" barrel, for a .223 I like a 20" H-bar type barrel like the bushmaster predator.