The 590A1 has a thicker barrel and metal trigger guard, but no heat shield, though I think the standard heat shield will fit on it, as there's a fair amount of clearance between it and the barrel on the standard 590's.
I have inspected them closely and they are very nice, but I am sort of interested in the Benelli M4, which I have been told by the factory, will be imported stateside, but in what form they don't yet know. I'd guess it'll have options for either a standard stock w/ 7-shot feed tube or pistol grip w/5-shot tube, like the M1. Mainly, I like it that semi's are less punishing than locked breech designs. But the Moss 590A1 carries a fair amount of weight, so it might not be all that bad. It's definitely the AK of shotguns: feed it anything, shoot it 'til it glows, use it like a baseball bat, run over it with a truck, then repeat until you get tired - it won't.