I'm fortunate in that I've got both. If I had to narrow things down quite a bit, the Montefeltro would probably be my pick. I've run many thousands of shells through it and it still looks and functions like new. I've used it on birds, sporting clays and trap with not a single glitch.
Even though I shoot a Browning 425 O/U for most clay games now, I probably shoot the Benelli a little better. It's the only shotgun I've been able to use to run perfect scores on sporting clay and 5-stand courses. It's kind of a bad time to ask me about O/U, since I've got a fit problem with my Browning right now. I've actually gone back to the Benelli until that gets worked out.
If you reload, the O/U is a lot more convenient. Also, when hunting, I do find it convenient to have two different chokes available.
If you shoot the 686 well, then I don't really think there is any reason to switch. The Benelli is a good shotgun, but so is yours. Besides, if you wanted to "switch back", you'd end up losing more money on the deal.