I'm stealing this question from CNN's Jack Cafferty who asked it on the air the other day.
My own opinion: neither is worse than the other. Both things are bound to ruin a relationship. Most marriages end over two things: infidelity or money, and it's easy to see why. A person who's dishonest about money is exhibiting the same character traits as someone who's not honest about what they're doing sexually. It shows a lack of trust, a lack of faith in the relationship and that that person doesn't have a level of commitment to the relationship that would allow it to work.
What's your take? Do you think your relationship has a better chance to survive cheating or your partner lying about how much debt they have?