Mei wins because Sakura can't use Tsunade's regeneration ability.
Even if she could, Mei would win low diff.
Sakura getting hit with Youton = gg. Lava isn't getting off her by itself, it's gonna be there, so unless Sakura tries to remove it with her hands (which will melt in the process), she cannot regenerate from it.
She's not dodging Mei's big-ass Suiton or Futton.
In this scenario, Mei stomps.