Wilson Combat (Scattergun Technologies) buys unfinished 870 Express receivers and barrels form Remington and then tunes and finishes them to their own specifications. Are they superior to an 870P? I don't know that I would say superior. Definitely more expensive and more nicely finished, but not necessarily superior...
Personally, I would say buy either a new 870 Express or better still a used Wingmaster on the EE, and send it off to Casey at TacOrd for a Vang comp treatment, some nice ghost ring sights and TacOrd's special Teflon finish. You will spend less than a new Scattergun and have a gun that is definitely superior to either of the two you mentioned.