The lawyer favored the punishment of life in prison, and the banker favored capital punishment (execution).
"Capital punishment and life-imprisonment are equally immoral; but if I were offered the choice between them, I would certainly choose the second. It's better to live somehow than not to live at all."
The lawyer admits that both forms of punishment are equally immoral, which means that neither punishment is inherently better than the other punishment. However, in his opinion, the lawyer essentially argues that some life (in whatever form it may be) is better than no life at all. I guess if I was given the choice, I wouldn't choose death either.
The lawyer's opinion infuriated the banker, and the banker laid out the parameters of the bet for which the story is named.
"It's a lie. I bet you two millions you wouldn't stick in a cell even for five years."
What I've never understood though is why the banker proposed the bet in the first place. The debate was life in prison, and the banker offered 5 years. The bet throws in a completely new time variable into the discussion. Plus, the lawyer now has the opportunity to prove his point and make money doing it. Normally any amount of time in prison doesn't net the incarcerated person any financial payoff.
What I also don't understand is why the lawyer didn't take the bet as is. Instead of taking the five years, the lawyer upped the ante to 15 years for no further money. He's either desperate to prove his point, or he just doesn't like being around people.