I don't think Soresu would be better against Maul, you can see in TCW he still got *** kicked by the half cyborg Maul, without using double blade lightsaber.
I'm not saying that Obi-Wan picked Soresu to be the future answer to Maul's Juyo. I'm saying that Obi-Wan saw that Ataru's flourishes and emphasis on wide open attacks left gaping holes in your defense, therefore he opted for the much more defense oriented Soresu.
Obi-Wan Kenobi originally specialized in Ataru, trained in the form by Qui-Gon Jinn. However, upon witnessing his master's death due to Ataru's lack of defensive capabilities, Kenobi took up dedication to Soresu as a defense against the Sith techniques that brought down Jinn. Kenobi's resulting skill in Form III stood him in good stead against masterful gunslingers such as Jango Fett, or during the chaotic firefight on Geonosis. His tight defense and energy-efficient moves enabled him to pull through and survive. However, when confronting Dooku, Kenobi's skill in Soresu was insufficient to effectively counter Dooku's mastery of Form II lightsaber combat. Dooku's elegant style worked away at Kenobi's defense; many of Kenobi's cutting parries missing Dooku's attacks entirely, while the Sith Lord's feints confused and forced him to retreat. Kenobi was ultimately brought down by light, though damaging cuts across his left arm and leg.
Quoted from the Wookieepedia page for Form III: Soresu