Ignore everyone else, and listen to me alone!
Both are good. If one or the other is substantially cheaper and in good shape, get that one.
If they are the same cost, pick whichever one feels best in your hands. If you can, before you buy, find some folks who have them and get them to take you shooting, so you can try them out.
And don't tell me Jedi mind tricks don't work on you! You'll hurt my feelings! ;)
I believe the Mossberg 500 will probably be cheaper, if you get a stock weapon. Get one that is gussied up for “tactical” use, ie, an ugly black gun, and it's going to cost more. Remington's 870 might possibly be a bit higher quality, or so I've heard. I've owned one of each, but a long time apart, and I'd be happy to have either kind. Especially right now. Mine was gussied up with the folding pistol-grip stock in black plastic, but the original forearm grip in beat up walnut. I think someone ran out of $$ while playing with it. I got it in a three-corner trade long ago, and it's long gone. I liked them both.
I love it when a post starts like this.
I am not weak minded but Jedi Mind tricks tend to work on me!
“I am young and a bit naive when it comes to firearms but am interested in a good home defense shotgun. As I am well aware many of you have experience in this area I was wondering what recomendations you might have to Remington 870 express vs the Mossberg 500?”
I picked up an older (almost as new) mossberg 500AB with a slug barrel for $125. I suspect the previous owner put a box of shell through it and stuck it in a closet for 25 years. It holds 5 +1 which is more than enough for home defense. Try to find an 870 for that price.