Hello awmp. I had the same question as you before I settled on the M2 Tactical. My M2T only has about 200 rounds through it, but it shoots like a dream and is lighter than the M4 which I have handled. Most M4 owners on this forum will tell you that the M4 is so much better than the M2, but I don't buy it. The M4 is an outstanding combat scattergun, I'm not denying that, but after watching Tom Knapp and his fellow exhibition shooter buddy with their M2's and the US Army Shooting Team members with their M2's, I came to realize that I don't need anything "better" than an M2. It's inertia driven system has been around alot longer that the M4's ARGO system and the fact that most of the Benelli shotguns including the top-of-the-line clay and hunting models use the inertia driven system is a testament to how proven and reliable it is.
On the other hand, if you plan on mounting a bunch of heavy add-ons, then the M4 might be for you. I know of other fellow law enforcement officers who have been shooting M1's for years with mag extensions and surefire lights on them with no problems. Mine will soon have these accessories and I will test it with many rounds to see if I can dispell the "myths".
Hope this helps.