Vader wins. Obi Wan was still on the defensive the whole time and their knowledge of one another's forms evened out the playing field; besides, Obi Wan has better saber feats than Arcann anyway.
Also, Vader won't be as emotionally hindered against Arcann. He'll just see him as a random pest to be exterminated, not his former father figure. He'll still suffer from his sleep deprivation or whatever, but I'd still put him above Arcann.