2 months late, I know, but I was thinking about this MU and found your post

In either round, it really depends on circumstance (as most crossover fights do). Akaza has insta-regen, and as long as the fight is during the night and Sukuna doesn't have the knowledge & access to either Wisteria poison or a Nichirin weapon, Akaza is unkillable. Sukuna, on the other hand, definitely outmatches Akaza in terms of offensive capabilities, but curses can technically be killed by enough brute force (unless I forgot they disclosed that curses can only be killed with cursed energy).

If Sukuna can stay on the offensive for a full 12 hours until it's daytime, Akaza would eventually get killed by the sunlight, but if Akaza used cunning to weaken Sukuna enough, he might be able to kill him.