Both the 500 and the 870 is drilled and tapped for a scope mount on the receiver.
Both the 500 and the 870 are available with or with out the cantilever scope base.

They both use the same type of locking engagement. The Remingtons bolt locks into the receiver and the Mossbergs bolt locks into the barrel.
The Remington uses only one extractor where as the Mossberg uses two extractors.
The remington is available in black synthetic. The Mossberg is available in wood, synthetic, or Mossy Oak break up.
You can get 3.5" magnum versions from either company.
The Remington is generally a little bit smoother.
Either one will provide you with a life time of service provided you take care of it.
To be honest I'd just flip a coin and stick with it.
Just wanted to add that which ever one you choose I would
definetly go with the cantilever mounting system. It is much better to have the scope mounted to the barrel than to the receiver on a shot gun or any "rifle" that has an interchangeable barrel.