I have the 18.5" but have added a + 2 extension to give me a total of 8 rounds loaded in the gun. I chose the 18.5" because I could not find the 20 anywhere last year, and I'm glad I did. With the heavy barrel on the 590a1 it gets heavy after a while. If you are going to take it out and train with it the extra weight would be noticed quick.
I like my ghost ring sights they are quick and accurate. They would really be appreciated if you wanted to take a longer shot with a slug. I use a 7 shot velcro side saddle from skd tactical that I like real well. Picture below.