Tough call for me. The 870 has been my go to gun for the last 20+ years. It's been though hell and back, cycled many 1000's of rounds wihout a hickup and has never let me down.
I then picked upo a used 500c, that too has operated flawlessly. I don';t have as much trigger time on is as I have my 870 but I have not has a single issue with it mechanically My 835 is still sitting in a box waiting be be refinished so I can't comment on that one, it was bought specifically as a project gun.
The ergonomics of the safety and slide release are difficult for me to perform as reliably as my brain is still wired for the 870 controls but that's an operator issue, not the equipment and I just neeed to get more practice for the Mossy controls to become automatic for me.
I'm not sure one can compare the two in terms of which one is better, both have earned a reputation for reliability. Some people let the price tag influence them as to which is a better gun but from my experience so far, I would trust my life to either one.