I own both.
My 12ga. M500 has a rifled barrel and is used for deer hunting only. It's had a total of, maybe, 30rd put through it. 30rd. isn't much of a round count to assess reliability, so all I can really say at this point is that it's been flawless. Take it for what it's worth. I have no real complaints with it at this point. The trigger is a bit heavy, but can be worked over if you're into tinkering. Fit and finish is what I'd expect for a $300 gun. Cycles smooth enough. I don't regret the purchase and it serves it's purpose.
My 12ga. 870 gets the shit used out of it. It's the exact model you have linked. It has had one glaring problem with it since I bought it. Remington get a bit over zealous with that shitty matte coating they put on them now. The problem is/was, they got said coating inside the chamber, making for some very tight tolerances. It would load a shell just fine, but when you fired it and it expanded, it would wedge itself in there making it very difficult to eject. When I first go it, I'd have to slam the butt on the ground to get it to dislodge. I wasn't very impressed with having to do this, especially considering the 870s reputation. It loosened up over time w/ use, but I got tired of it, and went to town on it with some steel wool to remove the rest of the coating. It now cycles great. One other problem with the shitting coating is that it's not much of a coating at all. If you're not careful with it, it'll rust in a minute. Again, I'm not impressed with that. Remington could have done better, IMO.
Overall, I like both guns. They were cheap enough, and both serve their own purpose. My 870 has been the only problematic one, and though it wasn't really a big problem, it shouldn't have happened with some proper QC in place.
Anyways, that's my opinion. It's also my opinion that the pistol grip that comes with the M500 should go straight into the trashcan when/if you get it.