I happen to have a Mossberg 500 and a Remington 870. There are nice features about both and some features that some may like better than others.
If you compared the most common models or introductory level models liek the standard mossberg 500 and the 870 Express in their Home defense models, then the similarities would be:
-Both have 18.5 inch barrels
-Both have plastic trigger guards and polymer furniture.
-Both have different sighting options as different models in each manufacturers catalog but both of the basic models come with bead front sights.
-both come with blued finishes, The Remington has a bead blasted texture before bluing which makes it a more matte finish, while the Mossberg is a gloss finish.
-Both have dual action bars to help prevent binding and twisting when operating the pump.
-Both have similar bolt locking designs.
-The Remington has a steel receiver while the Mossberg has an aluminum receiver.
-The Mossberg slide release is located behind the trigger guard making it possible to operate the slide without having to move the shooting hand and it is easily operated right or left handed.
_The remington slide release is forward of teh trigger guard and requires a right handed shooter to move his weapon hand off the grip to operate it.
-The Mossberg safety is on top of the rear of the receiver, again where a right or left handed shooter can operate it easily. (It is not as easy to operate though if you opt for a pistol grip of any kind.)
-The Remington Safety is a button at the rear of the trigger guard,that is easy for a right handed shooter to disengage buta bit more difficult for a lefty.
-The Remington teds to ba a shade easier to take down for cleaning.
-The Remington has a single extractor while teh Mossberg has dual extractors.
- The Mossberg ejection port is slightly more generous and makes port loading a bit easier.
Both are good solid shotguns that have good track records with police and the military.
Shooting both side by side I found tha tthe Remington shot slightly larger patterns than the Mossberg. The Remington has a better trigger. The Remington is better at putting slugs at point of aim, but seemed to show a preference for Foster type slugs, While the Mossberg seemed to shoot Brenneke and Foster slugs about the same.
If you want to "Tac out" a standard 870 and are wondering if you can keep it within budget I would need to know what you consider "Tac'd out", and what features you consider "Must haves".