Brent has pretty much summed it all up....in fact, he completely summed it up! For me, the 500 with the short mag (5 shot) has proven the most comfortable for swinging/handling, and gives the most versatility with respect to changing barrels.
I have used/owned the 20 inch 590 with the full length mag tube, the 20 inch 500 with full length mag tube, the 18.5 inch 590A1 with 5 shot mag tube and the 18.5 inch 500 with 5 shot mag tube, ....and have tried the 20 inch 590A1 with full length mag tube. I find the 20 inch 590A1 (the A1s have heavy barrels) to be really muzzle heavy. The 20 inch 500 or 590 with the lighter barrels are OK but not as handy as the 18.5 barrels.
I like the feel of the 18.5 inch barrels and own a 590A1 and 500 set up as such. I find no practical reason why the 590A1 is a better HD gun and in fact, I normally carry the 500 at work. I am a detective and department armorer and have had zero problems with our dept 500s and 590A1s except for cracked safety buttons on our really, really old 500s that had a thinner checkered plastic button. I haven't seen any of the newer plastic buttons crack as they are thicker and have horizontal serrations. If the thought bothers you, though it is easy to change the safety for a metal one from Mossberg or Vang comp etc. It looks like you have the nod from Brent in Florida and you have mine from Vermont.....that covers the really hot and muggy and the really frosty cold. Can't see how you can go wrong with the 500....and the plain jane 18.5 incher is reasonably priced, as is the field and security combo. You don't necessarily need super tacticool, you need a reliable gun you can load and shoot quickly and accurately.