What transpired at the FPA's Success Forum was sort of a ying-yang message. On Sunday morning Bodie addressed a packed room of about 200 of advisors, and basically told them some of their professions "articles of faith" are defective.

"Stocks are not safe in the long run," he told them. "Beware using probability of a shortfall as a measure of risk."

On Monday afternoon Siegel, speaking before a much larger audience, used stock data dating back to 1802 to show that stocks have been better investments than bonds for a period spanning two centuries.

From 1802 to June of this year, Siegel said, stocks have provided an after-inflation return of 6.7% per year, compared with a 3.6% return for government bonds. He also pointed out that through history stocks have been volatile, but have unfailingly reverted back to a mean.

"There has never been a 30-year period where bonds have outperformed stocks," said Siegel, who could not be reached for comment for this article.

Whom is an advisor to believe?

Paula Hogan, president of Hogan Financial Management in Milwaukee, who invited Bodie to speak at the Success Forum, feels advisors are more receptive to Bodie's warnings after the volatility they've experienced in recent years.

"The current generation of investors and their advisors grew up in a world where the paradigm was, 'Stocks may be volatile, but if you hold for a long time you win,'" she says. "It took the bursting of the stock market bubble to look at the other half of the idea, which financial economists have always been aware of, that stocks are risky even in the long run."

According to Bodie, the contrast in his and Siegel's messages may not be as striking as it appears. Bodie still believes stocks are good investments, after an investor decides how much he needs for retirement and locks in those funds through hedging and inflation-protected products.

"(Siegel) agrees that if an investor wants to lock in a return without risk, the way to do it is with TIPS," Bodie says. "On the other hand, he doesn't think stocks are quite as risky as I might say they are."