Interesting bets - if you ask me I'd lean towards both main characters getting killed off or going to jail in the final episode.
Dexter has a few people that could take him out between now and the finale - the blonde chick that likes poisoning people, the young guy that he's trying to mentor, or any one of the other characters in the series could discover who he is and catch/kill him too.
Walter White is also at risk of dying off, even if he doesn't get killed by something related to his occupation, don't forget he's still got the cancer so I think you chose wisely on that bet he could very easily get finished off by the end of the series.