EdInk.... Here's my disclaimer... this is just my personal opinion, consider it as you will.
Any of the models you're considering are excellent shotguns. The 500 is more versatile for HD and sporting (with additional barrels), whereas the 590 is more dedicated to HD.
I have two Mossbergs, an 18.5" 6-shot Mossberg 500 Persuader and a 20" 9-shot 590 Spl. Purpose. Both are blued and I like them both quite a bit.
This photo gives a size comparison of the guns in stock form: (Coach gun just happens to be in this pic also because I have and love that gun too.)
Just looking at the 500 and the 590 in the picture, it would seem that they wouldn't feel much different, but they sure do to me. See all that mass up front on the 590 as compared to the 500? It makes a very noticible difference in handling the guns!
If I had to choose between the 18.5" 6-shot and the 20" 8-shot 500 or 20" 9-shot 590, I would go with the 18.5" 6-shot 500 because I much prefer the handling of the gun. The 20" with full mag tube, while just 1.5" longer... when combined with the additional mag tube length/weight and extra out-front weight of the additional shells, and the bayonet lug weight, and heat shield weight, feels noticibly much less nimble, swings less well, and the forward weight balance is significant.
So, while I like the idea of 8- or 9-shot capacity, personally, my choice (having both) is for the shorter, lighter, more nimble 18.5" 6-shot 500! The gun just shoulders, swings, and handles so beautifully!!! By comparison, the full dress 20" 590 feels like a boat. But I do like the 590, because hey, it's a Mossberg 500!, but, ya know...
The argument could be made that the extra weight helps soak up recoil which to a degree is true. But I don't find the difference bothersome at all with 00 Buck. As with all things, though, there is always a price to pay for whatever choice you make! It is nice having a nine-shot, and an eight-shot, and a six-shot, and a double. None of them are the "perfect" choice.
Having a number of different shotguns is nice. But for the person who is thinking about perhaps having only one... that's where I'm thinking the 500 probably makes more sense than the 590. If you can have more than one, then, wonderful. Get an 18.5" six-shooter and a 20" nine-shooter and.....and.....
Note that the 590A1 has the thicker (and heavier) barrel vs the 590 SP. Is a thicker barrel needed? I think only if you will be really knocking the gun around and very likely banging the barrel into things. Think "service gun" vs. personal civilian weapon. Personally, I have no need for a thicker/heavier barrel.
This is how I have my Persuader set up now. My preference is for the look and the feel of wood furniture on polished blued guns, but plastic furniture and parked finish is arguably more utilitarian and durable if a gun is to be subjected to a lot of hard, rough use. I'm not so hard on my guns.
How do you want to use your guns? HD only? HD and possibly some sporting use? Toss 'em around and beat the heck out of them or take good care of them?
Another option, of course, would be the 18.5" 590A1 which would give you some of the better handling with some of the 590A1 features.
A metal safety button (Vang is great) is easily put on a 500.
Keep in mind that most of the premier shotgun manufacturers in the world have found that aluminum receivers and polymer trigger guards serve very, very well indeed for the vast majority of users. You can go heavier and beefier if you want to (steel receiver on the 870, steel trigger guard), but you very well may not need to.
As for the heat shield, personally, I have no need for them and removed it from my 590 SP. Barrels get hot with long continuous usage, but it's not difficult to handle the gun in such a way as to not need a heat shield. I'm just not into the look nor the added weight, myself. I wouldn't mind going without the bayonet lug as well. (Can you see why I like the 500 better?
)Here's how my 590SP looks without the heat shield and with wood furniture:
