My vote is for the 870. They point much better FOR ME than the Mossbergs. I would rate them as equals in reliability, and availability of aftermarket accessories.
I run an 870 express with a 21" Vent rib barrel for 3 Gun Heavy Metal. I added a Remington 4 shot extension and opened up the loading port so I could shove shells in as fast as possible. Thats all the mods it needed, I believe in keepin a shotgun as simple as possible. With birdshot and buckshot it shoots where im looking, and I have no trouble keeping slugs in an IDPA target out to 100 yds with just the bead. It has been flawless in about a dozen matches and a few thousand rounds fired in practice.
Personally, I would steer clear of all the tacticood accessories. It just something to fail. Maybe a light and one of the velco/elastic side saddles if you just can't live without it.