Idk if this means much, but in terms of AP, Ven scales to Sora and Aqua specifically on his profile while Roxas scales to the other Guardians of Light. Dunno if theres much of a difference, if any at all there.
That aside, if we look between their powers and abilities, resistances, and techniques that are applicable to this fight, Ventus has Roxas beat in terms of what he's able to bring to the table. Ventus has access better/more magic. Idk if command styles count for the KH3 version as we the player never got to use them, but lore wise I dont see why not as Aqua was able to in KH3. If he does, they give him a boost both in stats or at the very least speed, and of course versatility and how he fights, he has better mobility via the keyblade armor etc. While they both have resistances to alot of each other's arsenal, that doesn't mean they don't take any damage whatsoever.
I don't wanna say Ventus outskills, but in the short timeframe of the beginning to the end of BBS, Ventus grew from being someone that wasn't very strong compared to other characters we've seen from KH1 up to that point to being able to defeat Vanitas with the Incomplete ¤ç Blade, who's comparable to Twilight Xemnas, who took endgame KH2 Sora and Riku to beat, while Roxas after a year of fighting (far longer than the events of BBS) was around pre endgame KH2 Sora, and their fusion is what made endgame Sora...endgame Sora. Ven was able to close a gap as large as that in a short time.
As for who they fought in KH3, we get Saix and Xemnas for Roxas (alongside Sora, Kairi, Xion and Axel). With Xemnas being a top tier, though in that fight he casually restrained Sora and Kairi, fought the other 3 with Saix, then causally took Kairi and left while unharmed and unphased. And then after that, the 4 of them minus Kairi vs Saix. On Ventus' end we have him and Aqua fighting Vanitas and Terranort in a 2 v 2 evenly or slightly on the losing end before Sora stepped in. With Vanitas casually blitzing Sora, Aqua, Donald and Goofy in the cutscene prior to the Aqua fight and preparing to take all 4 at once, and being comparable to Aqua in their actual fight. Ventus and Vanitas' clash shattered Aqua's barrier completely if that means anything. As for Terranort, he was strong enough to blitz an off guard Ven and Aqua and one shot off guard Ven (not an anti feat cuz off guard), one shot an on guard Lea who blocked, toss aside Sora casually, clash with and overpower a KH3 Lingering Will, beat Re:Mind Sora even with Lingering Will intervening to help...
And then at the final battle Aqua and Ven were even or just barely losing to those two. When Sora came, they managed to beat Vanitas and Terranort. Them beating those 2 compared to Roxas' team not phasing Xemnas and 4 v 1 Saix compared to Ventus and Aqua 2 v 2 and then 3 v 2 with Sora and beating both enemies. Not saying Roxas wouldn't be able to do it if we switch him and Ven, but the feats and scaling are in Ven's favor ontop of what I mentioned before
So based on all that I'd say Ventus more often than not. Roxas has Nil, which he can use to basically ignore Ven's durability, but he'd still have to land an actual hit. Which is definitely possible. I just see Ven winning more often than not