If you ever get a chance, field strip a Remington 870, and then take apart a Mossberg 500. Once you do, you'll be 100% convinced to buy an 870. Mossberg developed the most idiot disassembly process that could have been conceived for a pump shotgun.
Beyond that, quality is what you make of it. Remington cuts corners in certain areas, Mossberg cuts them in others. If I had the choice between the two, I'd take a Remington any day of the week. I've had numerous Mossbergs malfunction, both in the field and in combat conditions. I realize that many people have Mossberg 500s that run just fine, but I've been burned by Mossberg too many times. I have also had Remingtons malfunction on me in training, but their malfunctions are not to the level of the Mossbergs, and the Remingtons we have that malfunction do so because of maintenance issues and high use.
If a tactical shotgun is really what you are looking for, Benelli is the place to go. If you don't want an auto, the Benelli Nova/SuperNova is a great option. If you want something else, then the Remington 870 is the next best option IMO.