Number of shots, and how ever much 4.5" of steel weighs on the barrel/mag, that's the difference.
I've been prancing around the house lately with a 14" 590S with a Magpul butt-stock, and a 18.5" 590A1 (Magpul both ends) to decide which one I'm keeping. The Shockwave is lighter (quite a bit), but that's really about it, you aren't going through a door clean with either.
If you can find one, a 18.5" 590A1 with removable chokes, it's what I'm keeping. The pattern with a mod choke, and Fed LE 00 BS makes about 1" hole at 10 yds. Shotgun, or 25mm Cannon...
As for stocks, Magpul butt, and the plain factory fore-stock.
A 590S is pretty cool, buckshot mini shells are like shooting pistol cartridges, and the the slugs are 1200 fps, pretty much what the law shoots power wise. The factory mod works pretty good, so far the only problem the little bumper popped out, but only once, and I've put 300-400 shells through it. The 590A1 mini shell adapter is less than perfect, pretty good, but a bit unreliable in the cold, and it's a pain to unload. I'm going to remove that, and send it to a new home.