590A1 is a better gun, but you're not going to hear that from most folks on the Remington koolaid, as many are here. The 590 has dual extractors to the Remington's one, it is milspec and tested where the 870 isn't, the shell lifter is up and out of the way for loading and topping up, versus the 870's hanging annoyingly in the way. The safeties are also a world apart, the 590 has a perfectly placed tang safety easily operated with gloves on, and the 870 forces you to fumble around within a 1/4" or so of the trigger to put it on and off.
The 590 isn't perfect, but it's better than the 870. 870's a good gun too- the 590 just trumps it.