Who do you think won the bet?

I don't think that anyone wins the bet, really. The young lawyer ends up mentally broken, prematurely aged by fifteen years of complete isolation. As for the banker, he may have won the bet in a technical sense, but it's very much a Pyrrhic victory in that he's financially worse off than he was when he started.
Also, one could say that the banker's soul—such as it was—has been totally corrupted by the bet. When it seemed like the lawyer was about to win the two million rubles, thus plunging the banker into financial ruin, the banker got so desperate that he came within an inch of murdering the lawyer—but because the lawyer gave up the ghost at the last minute, the banker didn't need to go through with it. Nonetheless, the very fact that the banker seriously contemplated killing someone in cold blood indicates that he's been morally corrupted by the whole affair. In that sense, one could say that he's lost a piece of his soul by entering into this highly unusual wager.