One advantage of the pump is versatility...no minimum power level required for your shells in a survival hunting situation...not to mention working in mud, water, sand, etc. Another is the fact that an extra barrel (if one is needed) is much cheaper, and it tends to be a bit easier to swap parts, etc. and get the gun back up and running if required, and there are millions of 870s out there to cannibalize parts from. You can easily buy two standard 870Ps for the price of a single M1 (M2 now) new.
My personal preference for a pump is based on decades of use...my hand just fits an 870 like petting an old dog. My Benelli M1s are also valued friends and I do not plan to get rid of them. They are damned reliable, and you make a good case for selecting one, but no way they can compete with a good pump if I could use only some "dipstick" motor oil and a t-shirt for cleaning.
My choice from your list? The 870P Max...but I would get a shorter stock and an extra, longer barrel with screw in chokes. (Should be interesting shooting the GR rear with a bead front sight!)