20" will not get you a gun with better accuracy than a 16" with the same quality barrel. If you use optics (scope, red dot, or holo), it won't make a difference. If you use iron sights, then the 20" will give a marginally better sight radius, which could mean the shooter might have better accuracy, but the extra weight on the end of the barrel may impair accuracy if it is not supported on a bipod or bags.
A 20" barrel will get you about a 5% faster muzzle velocity, which means a marginally flatter trajectory or slightly longer range. The one key thing to consider is the 2700 fps point in the bullet's trajectory. At faster than 2700 fps, a 55 gr fmj bullet will tumble and fragment when it hits flesh. Slower and it is more likely to pass through. For a 16" barrel that point where it transitions below 2700 fps is about 150 yards and for a 20" it is about 200 yards. Bare in mind that it is really only a SHTF defensive consideration, because if you are hunting you will be using SP or HP rounds as FMJ are illegal for hunting many places.
Unless you are sure you have a specific need to go longer, get the 16". Worse case you can always replace the barrel in the future or get a different upper.