Izuku and Katsuki with Full Cowl 100%, huh?
Let's get this out of the way, first and foremost. They both the same AP and speed, presumably, so that wouldn't be any trouble for them. And, both of them have access to the Detroit Smash, which can cause air pressures strong enough to overpower Nine's storm, so their physicals are obviously greater than that.
(On that note, they should be "At least Large Mountain level" because they used the air pressure of their attacks to disperse the storm, with the air pressures being weaker than their punches.)
But, that means literally nothing because Katsuki has Explosion, which is more destructive and has a better range than Izuku's attacks. He doesn't have Blackwhip here, I'm pretty sure the movie happens before he unlocked it, but he does have better control of One For All which could carry him through the battle. Explosion is just too good for Izuku to overpower, especially since they are equal in power. Eventually, Izuku would cave in, or maybe Katsuki's body would destroy itself before then. I don't know.
Basically, Katsuki wins because he has Explosion along with One For All, which is possibly more powerful than he and Izuku are physically, because One For All boosts the user's Quirk.