My HD 870 has a 6 round magazine, backup has a 7. Both are loaded one round under. The extension on the shorter one adds 7 oz at the muzzle, before the 6 oz or so for two shells. That's a full pound of weight right where it counts.
Nice for recoil control(What kick?) but ti's slower.
I also use another 870 for defense, a more traditional model with a 21" turkey barrel, no extension.
While I'm a big lout, strong for my age, and well trained, the last 870 mentioned is faster than a rattlesnake in my hands.. That's something to consider. The others I can handle, from practice and training, but the 4 round shotgun is faster.
HD crises are not going to use as much ammo as the Battle for Hue. If 4 rounds will not handle the situation, more are not likely to either.
How many of us have other adults who may need to use the shotgun at a terribly crucial time and lack the upper body strength to use a 9 lb, muzzle heavy weapon?
Everyone should very carefuly evaluate whether or not the extensions are right for them.....