This will be for a scout style rifle.
From my own experience, I vote for the 18" barrel with no muzzle device. Unless you just want to add one later, which is always an option.
If you happen to be looking into the Ruger Gunsite Scouts, or more recently named Ruger Scouts, the early versions came with barrels actually longer than listed on the spec sheet by Ruger. Both the early 16.5" and 18" GSRs* actually came with barrels at 17.25" and 18.75" long (measured with muzzle device removed).
My particular GSR was bought in early 2013 with the spec'd 16.5" barrel, but if you slide a cleaning rod down the bore with the bolt closed, make a mark on the cleaning rod, then remove that rod and measure . . . 17.25". Flash hider removed, of course.
So, if you did come across an early GSR with 18.75" barrel, it would be dang near the same length barrel as in the Steyr Scout in .308.
For what it's worth, I removed the flash hider on my GSR after the first time I took it out shooting. The few times I've let others fire it, the muzzle blast out of that bare 17.25" barrel is not worse than being beside a 16.x" barreled AR in 5.56 with a muzzle brake being fired.
*Note that Ruger's website currently shows barrel lengths of 16.1" and 18.7". Which pretty much indicates that Ruger makes the new longer scout barrels the same as they did back when they were called 18" barrels. Yet, the 16.1" barrels are certainly shorter than the old 16.5" (actual 17.25") barrels.