Hey Element 666,
Each Shotgun has its individual strength & weakness (this post is the endless flame war of which is better a Chevy Vs. Ford!)
Rem 870 - My own personal preference - Almost all steel parts (including receiver). A Ba-Freaking-Jillion accessories available (the Rem 870 is the 350 Chevy when it comes to someone makes anything you could have EVER dreamed of for it!) The Rem Expresses will need to have the "dimples" on the mag tube removed (these are to prevent placing a mag extension on non-police guns (bad "P.C." points on Remington for this!!). A gunsmith can smooth the dimples with a tube press in about 3 minutes flat and you are then good to go (cost me all of $20)! Rem puts the safety on the trigger guard, so more stock options (like a folder or pistol grip stock) will work well. The shell lifter is solid, so it keeps most of the crud out of the action, but if you get it really fowled it will take more effort to clean.
The 870 safety is not ambidexterous, but to me that is a so what (I and everyone on my team are all right handed). I grew up shooting a 870 so it is a natural and great fit for me!
Mossberg 500/590 - More alloy parts (makes guns lighter), lots of accessories but just not as many as for the 870. Mossberg 500 does not have any magazine extension available (Most Mossbergs have are made with a one piece magazine tube, so what you have is all you'll get. Could be 5, could be 8 depending on which version of the 500s and/or 590s you selected. The lifter at the bottom of the gun is open, so you can get more dirt & crud in there, but being open the crud can fall out easier too. Some people find the open lifter is easier to speedload. The safety is a tang at the top of the receiver, so it is ambidexterous for right or left access, but can't be easily reached with pistol gripped or more exotic stocked guns. Mossbergs do have dual extractors on the bolts so (in theory) they would be more reliable for extraction (due to redundency), but in my years of experience with both, I've found that 870's rarely have extraction problems so the point may be moot.
The U.S. Military has purchased many of both the 870s & 590s Govt. service is not a deciding vote there!
Having owned many 870s and 590s the answer about which is best is...
they are both dog gone good shotguns, so your best bet is to see what will work best for
your needs by trying each for fit and see which works best for your "groove! B.T.W. - I own both 870s and 590s! Love my 870s and my 590 Bullpup!
"BIGGER_HAMMER"h.gif