You will drop around 40fps, not tragic. If you are going to shoot mainly suppressed, and go with the 16" then you should look into a 1-8 twist. When you shoot suppressed typically you use heavier bullets, the faster twist will help to stabilize the heavy bullets. The faster twist also helps to better stabilize the bullets when they go transonic, this gives you better long range accuracy if needed. Long story short the 16" will do anything the 18" will do, so it comes down to your personal choice.