This is tought.
Both resist time manip, so let´s just leave that at a side.
Graviza and Zero Graviza will likely also be useless as Mario resists gravity manip-
Sora will lead by just using his Keyblade skills to hit Mario with it, but Mario can also keep up with his hammer.
Ars Arcanum and comparbale techniques? Mario just jumps upwards or uses FLUDD to dodge it and takes advantage of his vulnerability to stomp on him.
Flowmotion? Okay, that thing is actually tough, but Mario is known to often jump on walls in a similar fashion, so he can likely keep up by other of his jump varieties like the Spin Jump and the Blazing Wall Jump (Which basically allows him to jump around without even needing walls, acting as a sort of pseudo flight.
Sora also has the elemental spells (Firaga, Blizzaga, Thundaga, etc.), But Mario can reflect back most of them with his Cape Feather (Including Sleepga as it´s just gas)
He also has Cappy, which can easily possess him, but Warp can one shot him.
Now, for Mario:
Mario is likely to use fire attacks, most of which are proyectiles, which can be taken care of by using Reflega, but if Mario takes too much damage, much like Sora, he can not only heal himself, but also use special moves like making a forcefield to make himself invulnerable to direct attacks until being hit 6 times by anything,
How it can go?
The only hax that can work here is Sora´s sleep manip, which Mario easily nulls with the cape, and so the match ends up in them actually fighting rather than just a "who haxes the other first?"
Mario is a guy that is likely to try to dodge a lot and then attack, then repeat, while Sora is more straightforward and will try to just hit him with his Flowmotion attacks, eventually leading into Sora and Mario charging up their most powerful moves, leading into Mario making himself Lucky Cat Mario to further enhance his mobility and then make copies of himself to make it tougher for Sora, but they can be destroyed easily with a Firaga Burst.
Sora can also use Salvation to heal himself while injuring Mario a lot should he attemp to recklessly try to stomp him.
Worst-case scenario, Sora activates Second Chance and uses Curaga, he is even invulnerable while using it.
They are very balanced, but Mario has the advantage via better invisibility potential and Cappy, which can easily make Mario win via incap, so I vote for him.