I've had a Mossberg 590 Military for some time (extended tube, heat sheild, bayonet lug). I like it, but my brother has it now, because I got a Remington 870 Police.
The 870 just feels better. Tighter, better finished, the action feels more positive, and it mounts and points better (for me). Along with many, I prefer the location of the safety on the Mossy better, but that's it.
I won't be selling the Mossy, but I'm much happier with the 870. See if you can play with (and shoot) both before you decide. I'd always pay more for what I want, even hundreds more.
Let us know what you get and how you like it.