I’m guessing your 16" is chrome lined and the 18-20" will be stainless/precision? My 18" Noveske (stainless barrel) does shoot most anything more accurately than my 16" Noveske CL (groups run about 50% better).
The 18" barrel also pushes the same 77 gr SMK load a good 100 fps faster. Ergo between these two specific barrels, one CL and one stainless, the 18" SS is better for both distance and accuracy. Some of this could be related to the difference in weight - heavy guns are just easier to hold steady. As to going 18" or 20" for a precision barrel, I don’t really have an opinion - but assuming both were built for accuracy (over hard use longevity), I doubt it will make much of difference.
If you’re looking at long range you should look into actual velocities of the ammo you’re planning to use.
I found 40-60fps difference with 77gr ammo between 16" and 18".
After chatting with Noveske on my build, I went with a 16" midlength as it offered the correct dwell, weight, and velocity I wanted.
Aside note, Noveske and KAC do not use rifle length 18", they intermediate. I know rifle works, I just don’t think it’s actually “ideal”. Just like I know 12.5" and 13.7", midlength WORK, I know 14.5"-16" is ideal.
I lean towards the 18’’ myself here. There’s a balance between velocity, bbl harmonics, and portability. If it were a 338 I’d suggest a 20, but for a nice precision 556 18’’ has the best compromise between these factors.
Get a quality SS barrel. I opted for the Noveske SPR, and I’m very impressed. Get some quality 77 grain ammo, that also makes a big difference. I shoot 75 grain PRVI at the range due its price and accuracy.
Then invest in some glass that cost as much as your rifle, lol.