I have shot my friends 870 and like it. I have owned an 500C in 20 gauge with an optional slug barrel, and liked it, I have also had an 835 ultramag (early version) hated it.
I replace the 835 with a Benelli Nova and have not looked back. The Nova is just as affordable as the Mossberg, and is of equal or better quality (IMO).
The Remington and Mossberg will kill anything just as dead as any other shotgun, so don't limit yourself to just two options.