I think it's more of an even split. Maul does have more advantages than Kenobi, like strength, durability, stamina, martial arts, possibly tactical skill and just overall versatility. But Kenobi's use of Soresu is kind of tailored to circumventing these advantages, i.e putting up an impregnable defence, conserving energy while tiring out the opponent, leaving little room for openings ect. He's well suited to lasting against Maul.
As a quick side note: Maul also appears to have access to battle telepathy, and there's a slim chance that he could goad Kenobi into a rage like he has done before, although he may have learnt to refrain from doing that by RotS. Still, Maul actually clouded Kenobi's mind during their TPM duel telepathically and fed off of his anger in order to strengthen himself, so if there's a chance any of that could come into play it'd definitely throw things in Maul's favor. Pushing that to the side for a second, though..
Kenobi doesn't really have a way of defeating Maul in a lightsaber duel. Soresu is only good for exploiting gaps in defence and tiring out an opponent - and Maul's defence really isn't weak to begin with, considering he was doing just fine while on the backfoot against Jinn and padawan Kenobi together, and he doesn't even utilise any Forms tailored towards defence aside from Niman, which is a bit of everything. And Maul obviously isn't going to tire out before Kenobi.
Kenobi could try to push an offensive with Ataru, but Maul is highly familiar with the Form, especially if you subscribe to the theory that it's one of the prerequisite Forms he mastered in order to control Juyo. On the other hand Maul is completely tailored towards offence with his use of Juyo, Teras Kasi, Niman/Jar'Kai and again, maybe Ataru if you believe he has also mastered it (there's no better alternative). So that along with his superior stamina would more than likely allow him to defeat Kenobi after a ridiculously long duel. Unless you either see Kenobi finding a lapse in Maul's defence and exploiting it, or him winning through PIS/circumstances like usual.