Ok, well everything I was going to say has been said pretty much. I think the match goes to Marth because he outdoes Diddy in pretty much every department.
He does greater knockback, does more damage hit for hit, has better aerials, better ground moves, is a better edgeguarder, he is harder to gimp and has better close range options and defensive options overall.
If it weren't for Diddy's nana game this match would be a blow out. Probably 80/20 Marth or worse.
But Diddy does have his amazing Nana game to help him out. There is so much he can do and it really opens up tons of options and augments his game in many ways. Combos, shield pressure, out of shield options, spacing options, traps and a few other things.
Marth has his fair share of things he can do to mitigate the effectiveness of the nana game though. He can land on a nana with a fair which would keep Diddy at bay and pick it up. If his shield is hit with a nana he can JC fair from shield to grab it and zone Diddy at the same time. If Diddy glide tosses into Marth's shield then he gets up bed. And of course Marth's glide toss is nothing to scoff at. The distance is huge and Marth can net tipper smashes or a free DB if the nana hits.
Diddy's Banana game is amazing. But the fact that Marth outclasses him in virtually every other way is enough to give him the match.
60/40 Marth's favor.