I´ll go with Vasher, i just find him more interessting as a character and i think his character was more formed by his own decisions making his redemption more meaningfull (Marshs redemption doesn´t feel as strong to me since for the worst things he was controlled by Ruin and he tried to fight him from the beginning making the sins for which he redempts less severe). Also Vasher is a confirmed Worldhopper (not sure about Marsh) meaning he would have some idea what he would be up against in a fight, while Marsh doesn´t. Atium could be the deciding factor, but since Marsh needs it for his immortality i would guess he wouldn´t use it from the start giving Vasher an early advantage.