This match is tricky.
In terms of general skill, I'd say it's a tough call. Mori has a lot of advantages that Yu doesn't have. He has a generally higher level of versatility as well as better AD by far, probably more versatile technique mimicry, and an AP advantage.
Yu on the other hand is a far better generally. He has a very, VERY good analytical prediction at this point in the story that Mori generally lacks outside of upscaling. I won't call it "layers" anymore but the scaling is still quite ridiculous, and Yu is at the top of The Boxer skill chain by an insane margin. It is a match of versatility VS sheer practical skill.
I'd compare it to a mage who knows a bunch of spells VS someone whose mastered one single skill insanely well.
I think it's weird in the sense that the fight depends on the very start. Mori has a severe AP advantage and skills that bring that up much higher via amps, so one hit to a vital point can end Yu's career. The same goes for Yu and his dura neg, though. One hit to the heart or head and Mori will also be out of commission.
Now, we know Mori in this key is very immature. He notably gets very frustrated at losing and underestimates those who are "weaker" than him. He has a focus on "weakness" and "strength" that display a detrimental level of arrogance.
Yu on the other hand doesn't have this. He doesn't consider "weak" or "strong", and he has no innate sense of arrogance that causes him to hold back unless he has some other purpose (like for example, entertaining a crowd in a Boxing match). But since this is not the case, he has no reason not to go straight for the kill. This puts him a step above Mori, who won't immediately go for his strongest moves because he looks down on Yu for being so blatantly weaker than him.
So, the person who will likely get in the first death blow is Yu. But then comes in the problem of Mori's instincts, which allow him to avoid death blows and counter with even more speed and efficiency, even when he's nearly fallen asleep basically.
But then this brings up the point of Yu's enhanced perceptions, which would likely allow him to avoid a counter (which would work because Mori's development is not completely instantaneous), as well as his prediction which would likely negate an instantaneous counter.
Which then brings them into the 2nd stage of the fight, where they've now both registered each other as an actual threat.
Yu will probably try and zone Mori. He has far more experience in a ring-like environment, so things like distance, timing and etc will come a LOT more naturally to Yu and allow him to set up his "monster" stance from a distance, which is a FTE blitz-worthy amp that even blitzes people with similar perceptions to Yu. Mori needs to be very wary of this, because Yu is always aiming for death blows.
Mori would likely respond to this using one of his ranged attacks, such as say Kick of the Blue Dragon or Jin Hoechook off the bat. But the thing about both of these moves is that they take a bit longer to set up than Yu's monster stance, so in that exchange Yu has the advantage. Mori's instincts will have to save him again, but in that instance he can't dodge and attack at the exact same time, so that will probably throw him off and give Yu a better instance to attack.
By this point Mori probably would've started to adapt, using one of the other skills he knows in his repertoire to defeat Yu as his other, bigger attacks fail. In which comes down to the trickiest bit of the fight- Who lands the final death blow? Yu or Mori?
That's where I think the fight ultimately comes out to be an inconclusive. Thinking critically about how both of the characters act in a situation like this, as well as their general environment, it's likely the fight would play out in the steps I laid out. Which makes the outcome very ambiguous, as it all comes down to who you think will land the final blow that decides it all.
Someone could bring out "well Mori would just adapt!" but i find that not very likely because the fight is going to be a very fast paced battle. As ive illustrated Yu is not like Mori. He does not consider right or wrong. Weak or strong. So if he has the ability to kill, he will exercise it in any way he can as quickly as he can. Yu wouldn't drag the fight out to make Mori suffer like he did against Santorino.
So Mori and Yu will be within that quick exchange of killing blows, forcing Mori into a situation where he doesn't have the chance to adapt because a SINGLE wrong hit can kill him. Irregardless of how good your adaptation is in any scenario, if you potentially die instantly, it isn't going to help you.
And since Mori at this point in the story isn't used to battles that basically are life and death, where even the SLIGHTEST mistake can mean the end, I doubt he will have enough time to adapt. Yu however knows the risks he takes going into any fight, and he is used to a fast paced battle where there is no room for even minute error or else it's just over.
So in the end, my vote is inconclusive. At the end of a fast paced, life or death battle... both fighters are left waiting as one of them makes the final death blow. Mori hasn't enough time to adapt, and one blow means death. But Yu is running right into the throes of death as well, and Mori hitting him in the wrong way once could also mean its over. I think in the end, it all comes down to that, leaving the fight mysterious... ambiguous.
