What David said.
Unless you're going for an SPR precision setup like a Mk12, there is no reason to go with an 18" barrel. An 18" barrel is only necessary for running hot loaded match rounds for long range use, like with the Mk262 Mod1, where you need the extra inches to get the velocity up, since the 77gr 5.56 OTM load goes transsonic in the 700m range. Army Rangers have confirmed kills at over 900m with Mk262 using Mk12 rifles. That's what those barrels are for.
For modern carbine use, anything over 16" is useless if it's not specifically for longer range precision.