The SPDR Gold (NYSE: GLD) ETF is up more than 240% since 2004. The S&P 500 stock index is about flat over that period.

Gold has unquestionably been one of the best, if not the best, assets to own in recent years. While high-quality blue chip stocks that more than doubled earnings have stagnated -- Wal-Mart and Microsoft fit that bill -- precious metals stocks like Silver Wheaton (NYSE: SLW) and Yamana Gold (NYSE: AUY) have been staggering successes.

But when it comes to the question of what comes next, there is room for debate. It is a classic investing miscalculation to assume that what has done well in the past will do well in the future. History proves it's usually the other way around.

A recent Gallup poll asked investors what they think will be the best long-term investment. Thirty-four percent said gold; 17% said stocks. I asked Yale economist Robert Shiller what he thought about that statistic in an exclusive interview last month. Here's what he had to say:

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.