I own/ have an 870 for years now and love it. (Only shot it with my 18.5 inch barrel, havent tested out the 26 inch barrel i just acquired.)
I recently bought a moss 500, but havent shot it yet. Feels bout the same as my 870 while handling it. The only downside to the 500 is that i had to spend bout an extra 100 bucks on getting a tube action assym and hogie forend (870 comes standard with the slide action tube assym.) cause my forend has the pinned type of action bars. Read some people broke there forends with this style so i have a solid backup if mines ever break.
As for the 870, i dished out a few hundred bucks for a recent cerakote job. The finish attracts rust. Sort of speak lol. I wouldnt call it high maintenance but you do have to keep it nicely oiled/ greased for it to outlast you and your children's lifetime if you keep the parkerized finish. I keep the inside of the barrel nicely greased for both 870 and 500.
My 870 so far has a volquartsen extractor, magpul forend, streamlight protac 2/ aluminum mlok pic mount , magpul buttstock, magpul sling loop, troy 1 pt sling, and some carlsons aftermarket chokes.
My 500 has egw top pic rail, holoson 515t, cdm light mount, surefire light and thats bout it. Debating whether to cerakote the barrel professionally or try alumahyde 2 on it. Since the receiver is aluminum i might just rattle can the receiver as im not worried bout rust like the 870. But always good to have an extra "barrier" protecting the metal. Plus i like rattlecan jobs. Lol
I handled a benelli m4, m3, and m2. I like the m2 the most out of those 3. The m4 to me is heavy as fuck. It had the adjustable stock, idk if thats why it weighed alot?? Never looked up specs. M2 is cheaper by ~$500-700 when compared to m4s and it has the inertia gas system, so cleaner than the 1100 (which ive also shot.) And since the m2 is highly regarded as a competition shotgun theres a plethora of aftermarket items for it.