At what distance would the difference in barrel length come into play in regards to an 18" vs. a 20" ?

Assuming my greatest normal shooting distance would be 300 yards, I'm thinking the difference would be practically nill.

What about actuall energy delivered on target?

I am contemplating a AR type rifle in .308.  One is an 18" stainless barrel and the other is a 20" cm.

I've read that the 18" length is almost the quintessential length for the .308. Remington has 20" barrels on their tactical .308 bolt guns as well as 24".

I would say out to 300 yards the extra 2" of bar. will do nothing except make it heavier. I would also assume you won't miss the 2" until you start reaching 750+ yards. I think it is something like 60 fps/inch of barrel(I might be wrong though).

The reasoning behind the 18 being better is resonance and vibrations. My shooting skills wouldn't ever notice the .1 or possibly .05 MOA the resonance and vibrations will cause in inaccuracy.

My son, Jonathan.... catch my sig line below. Has a rifle built by Tactical Rifles with a 20" barrel. Length isn't as important as rate of twist. His is 1in10 and has no problem pushing 208 Amax past a grand.
The day I shot this pic.... Jonathan was making groups the size of a dime at 200 yards and complaining he was wide.....

If you are shooting 300 yards max, you could go to a 16" barrel.

I've shot out to 690 yards with my 18" using 168 SMKs and it did great.  

I'll be shooting out to 900 yards in a crop field this fall.

At 300 yds you could get away with using my 12.7" suppressed .308.

Most suppressed shooters go with 18-20" to keep the package as short as possible but still reach out to 1K yds.

I will be using a 20" 1:10 for my long range 308 and I use my 12.7" for shorter ranges when I still want heavier bullets.

6.8 spc....