http://www.mossberg.com/pcatalog/lawprc.htm
I don't know why the original poster's choice was only between an 18.5" model 500 or the 20" model 590A1.
I bought an 18.5" 590A1 with ghost ring sights. It has no bayonet lug, as the 20" does. I find the 18.5" swings and moves much better than the 20", but I lose 3 rounds capacity. If a weapon light is mounted then there'll be so much weight out front it won't swing as fast anyway, however. There's a slight difference in perceived muzzle rise with that extra weight out front.
Personally, I've found the 20" too long for me. The 20", vertically mounted between the seats in a Crown Victoria, hits the dome light when deploying. Unfortunately, I have no choice on how the gun is mounted in these work cars.
I've also desired a shorter barrel, either when slung, crossing fences, or when clearing houses, especially houses with narrow hallways. There is a trade-off in ammo capacity vs. barrel length to consider.
Again, this is my personal experience and won't pertain to others.
I really like the Mossberg. The ghost ring sights, safety, action release, weight, all combine to be a great gun.