BOTH are good, reliable shotguns perfectly capable of taking hard use if necessary.

I prefer the 870's steel receiver over the 590's aluminum receiver. The steel gives me more tactile satisfaction and theoretically should last longer than aluminum (but I've never heard anyone complain that their aluminum receiver wore out).

The 870 is a bit easier to reassemble after cleaning, in my opinion.

Good supply of "tactical" accessories for both, but you may have to look a little harder for the 590 as there are a lot more 870's out there and retailers therefore tend to give it more inventory room. (Tip: if you want a Surefire fore-end light for the 590, you need the model 623A01 to clear the bayonet lug.)

I like the 590's ambidextrous tang safety much more than the 870's safety on the trigger group housing.

Those are about the only pertinent differences in my opinion. I'll just give you the standard advice of handling both of them before you commit to a decision. You won't go wrong with either of them, though. Just get whichever one you subjectively like more.

I wish Remington would make an 870 with a tang safety or Mossberg would make a 590 with a steel receiver. That Remberg would be my ideal pump gun!