Do billionaire investors sometimes opt to press the proverbial easy button? You bet they do. Making money the easy way is preferable to doing it the hard way regardless of how much money you have.

The easy way of making money to which I'm referring is investing in exchange-traded funds (ETFs). With ETFs, you can buy a basket of stocks without spending the effort to research each stock.

Warren Buffett and Ken Griffin stand out as two of the most prominent billionaire investors on the planet. Buffett has led Berkshire Hathaway to reach a market cap of over $850 billion. Griffin founded Citadel, the most successful hedge fund ever. And both billionaires have pressed the easy button with one Vanguard ETF.

Great minds think alike

Buffett made an intriguing revelation about his will to Berkshire Hathaway shareholders in his 2013 annual letter. He said his will instructs that 90% of the cash inherited by his family be invested in an S&P 500 index fund. The legendary investor added, "I suggest Vanguard's."

He also put Berkshire's money where his mouth is. The conglomerate's portfolio owns dozens of stocks but also features two ETFs. One is the Vanguard S&P 500 ETF (VOO 0.93%).

Griffin's Citadel also has a stake in the Vanguard S&P 500 ETF. In addition, the hedge fund owns a large number of options on the fund.

This might be a little surprising. In a message to Citadel investors earlier this month, Griffin wrote: "In every investment strategy, we strive to create as many competitive advantages as possible over other market participants." Owning an S&P 500 index ETF might seem to contradict that strategy. Like Buffett, though, Griffin sees the wisdom in owning VOO.

Why this Vanguard ETF is so popular

The Vanguard S&P 500 ETF ranks as the third-largest ETF on the market with over $430 billion in assets under management. Over the last three months, its average daily-share volume topped 5.3 million.

Why is this Vanguard ETF so popular? The S&P 500 index it attempts to track is probably the most important factor. The S&P 500 includes the stocks of the 500 largest companies trading on U.S. stock exchanges. It spans multiple sectors and industries, providing the diversification many investors seek.

The index is rebalanced regularly. This leads to a "survival of the fittest" approach where the companies that don't continue to grow are replaced by those that do.

Many investors also recognize the S&P 500 has performed well over the long run. It has delivered positive returns over every 20-year period in history. The index's average annualized return including dividends is over 10%.

Several ETFs track the S&P 500. VOO's key advantage is its low cost. The ETF's annual expense ratio is only 0.03%.

Should you own VOO too?

Don't invest in any stock or ETF only because billionaire investors have done so, even ones as successful as Buffett and Griffin. However, I think most investors should own VOO, too.

This Vanguard ETF is an excellent starting position for beginning investors. Advanced investors can benefit from its diversification and long-term track record as well.

In his 2013 letter to Berkshire shareholders, Buffett wrote that an "investor who both diversifies and keeps his costs minimal is virtually certain to get satisfactory results." As usual, he was right.