RE: remingtion express or mossberg 500
To me, the Rem's action is a bit smoother (even in the express finish), but I own both. Lots of good accessories for both too including all the tactical stuff and cantelevered rifled slug barrels.
The biggest differences are the location of the safeties and action locks. This is a personal preference, and they are the first things I would tell you to compare for yourself.
My favorite safty location is where Winchester puts theirs - on the triggerguard in FRONT of the trigger, but to me, the Win's lack of smoothness is blown away by both Mossy and Rem.