A 20" barrel offers a longer sight radius
That will only be true if the sights are installed in a way that takes advantage of the barrel length. There are so many options. I have seen a lot that have the front sight mounted in locations other than the end of the barrel.
So its certainly an option, and one that I would like.
I mention it because I was looking for one with a 20 barrel and long sight radius, and didn't find any affordable on my budget. That was before FireArmageddon. Now - no way.