I have to agree with AZ; in theory, MK could have a ton of new banana tricks not known to people because he spent so much time with the match-up, but we can't bring theoretical stuff MK might be able to do with bananas to the table. If anything, the simple fact that Diddy can spawn bananas whenever he wants to ultimately gives him more control of them.
This.
Question: If simply holding an item first weren't an advantage then why would anyone be against items play in Brawl?
Answer: holding an item first is an advantage. The result is people point to random spawns as giving unfair/random advantages.
Overall its slightly in MK's favor. However I still firmly believe with more time it will become Diddy's favor. There are so many things that still need to be worked into my game for me to even think I'm close to mastering the character, not the least of which are: single nana locks, non-dash attack picks up, instant throws, shield bouncing, using more than just the F-tilt, etc etc. Despite lacking of all these things I'm still the best Diddy Kong in the MW and the only MK I probably can't beat is Judge, who I have yet to play (and think I would stand a good chance to win at least a game off of him).
Essentially, I have tons of room to improve with Diddy Kong and I am considered close to the top of Diddy's current metagame.
How much more room do Meta Knights have to improve at the top of MK's metagame? What things are have been discovered recently that makes MK better? What type of things can MK do that are limited in a players execution/technical skill like Diddy's nana lock (answer, everything MK does is easy to perform, so there isn't the same learning curves as Diddy).
Diddy is a character that went from 20th, to 14th, to 11th, to 10th, to 7th, to 7th, to 5th, and now currently 4th on Ankoku's list. He is a character that still few people play. He has far more potential than anyone is utilizing. He is also been moving on a curve up the ranks of characters, and that curve has yet to stabilize (Diddy is just a stones throw away from third on Ankoku's list). To assume we know anything about Diddy's potential, when even just 3-6 months ago many thought Diddy had peaked, would be niave.
MK vs Diddy is 60:40 at BEST. AKA, a not so great match-up. There is no debating it, and it's not opinion. Merely fact. Soon enough it will be obvious to everyone exactly why (the reasons Hanson so kindly brought up,) but that's all theory, so we'll just have to wait until MKs catch up to what they can theoretically do, OR until MK is banned and merely a disgusting feature in our not so happy past.
There is plenty debating it. People who think the match up is 60-40 or worse are in the minority and most of the notable players of BOTH characters believe the match up to be close to even.